Astrid and Hiccup's Relationship

Astrid and Hiccup's relationship throughout most of the franchise would best be termed as not always platonic best friends.

Astrid's feelings for Hiccup usually manifest themselves as extreme devotion and protectiveness, as she is always the first to aid him in any situation and the last to leave him, which Hiccup reciprocates towards Astrid, she was the last to leave him on his search for new dragons before Dagur broke out of prison and they moved to the edge. It appears to be an unspoken assumption that Astrid will accompany Hiccup on his adventures, more so then the other Vikings, and will usually invite herself along regardless. On the rare occasions that they are separated by mitigating circumstances, she is shown to worry greatly about him and is always overjoyed to be reunited with him. Beyond the romantic elements of their relationship, Astrid and Hiccup are the closest human friends that either has.

How to Train Your Dragon
At first, Astrid is shown to be disinterested in Hiccup. She considers him irresponsible when he tries to go against higher orders to prove himself and is particularly unimpressed with his inability to stay focused in the training arena. Unlike the rest of her peers, Astrid never ridicules Hiccup or laughs at him. Her vendetta against him seem to be on more personal ground and is fueled by suspicion and jealousy. Astrid grows particularly annoyed with him when he takes the upper hand during 'dragon training', suddenly showing far superior skills than she could ever hope to achieve despite all her hard work and determination. She also seems to be the only one able to see through the facade Hiccup puts up in the wake of his popularity. By the time Hiccup takes the final win, Astrid has grown beyond aggravated. Instead of confronting him calmly like she initially did when questioning him about his weirdness, Astrid follows him to the Cove and aggressively tries to press the truth out of him. Once she spots Toothless, however, her feud with Hiccup is forgotten and her first instincts are to protect Hiccup from Toothless. Notably, it's not the first time Astrid warns Hiccup as she told him to get down and out of the way when in the face danger during their training sessions. This time around, however, she physically attempts to protect him by pulling him to the ground with her and then telling him to run while she distracts Toothless. Hiccup is quick to intervene before anything serious happens. He attempts to calm both parties down and explain himself to Astrid who is thoroughly astonished by the revelation. Although it is unknown if she was actually running to the village, Astrid runs away in shock and fear, only to be picked up by Toothless and put in a tree. After finally agreeing with Hiccup to "let him show her" why he has a dragon, (having no choice other than fall out of the tree and die) she reluctantly gets on the dragon. But thanks to her attitude, Toothless doesn't listen to Hiccup's words to get down, slowly. Instead racing down, dunking into the water, and spinning in circles until Astrid apologizes. After he slows down, Astrid admires the beauty of the sky, especially as the sun starts to go down. She waves her hand through the clouds and even falls back so she can feel it in her hair. As they approach Berk, Astrid puts her arms around Hiccup's waist and leans her head on his shoulder, to Hiccup's surprise and then delight. As they go back into the air, she admits flying is amazing, and adding onto to it by saying Toothless is amazing. As they try to go back, Toothless suddenly hears a call and flies into line with a ton of other dragons, telling Astrid to get down so they aren't spotted. Once they are in the Dragon's Nest, Hiccup urges Toothless to get out for his and Astrid's safety, both Hiccup and Astrid slightly frightened when a Gronckle fails to get enough food for the Red Death and is promptly eaten. Once escaping the Nest, Astrid excitedly tells Hiccup they have to tell Stoick, but Hiccup gently takes her arm, telling her that they'll kill Toothless if the village finds out. She asks if he's really being serious about wanting to keep the Dragon's Nest a secret, something Vikings have searched for for thousands of years just to protect his pet dragon. When Hiccup firmly says yes, her face changes, realizing just how important Toothless is to him. She decides to support him, asking what he's going to do, to which he says he'll think of something. She then punches him in the arm, saying that for kidnapping her. As he rubs his arm, she grabs him by the shirt and quickly kisses his cheek, stuttering slightly as she says that was for everything else before running back home. In the Arena the next day, she meets with him before he goes inside, gently asking what his plan is. He tells her he has to try to stop this, but if something goes wrong then to take care of Toothless. She accepts, before asking him to promise her nothing will go wrong. He doesn't get the time to respond, as Gobber tells him it's time. After trying to train the Monstrous Nightmare and Stoick angering it by ringing against the cage, Astrid cries out for Hiccup, using her axe to force the arena open and then throwing a hammer at the dragon to distract it from Hiccup, running away from it herself. After Toothless is locked up on a ship heading towards the Dragon's Nest, Hiccup is angry and saddened, and Astrid doesn't make him feel better when she states the obvious (he's lost his tribe, father, and best friend) but slowly turns it into an almost pep talk/motivational speech. She asks him what he plans to do and he responds with "I don't know, probably something stupid" when she tells him it's a good start but he's already done that, he responds with something crazy. When Hiccup introduces the dragons in the arena to the other Viking teens, she shows her support, (probably being the main reason they go in the first place) and when Ruffnut tries to flirt with Hiccup, Astrid pulls her away by her helmet. During the battle against the Red Death, Hiccup saves Astrid when the Red Death tries to suck Stormfly into her mouth and eat her, before Hiccup has Toothless shoot a plasma blast at the Red Death, Astrid falling to the ground before Toothless grabs her. She smiles up at Toothless and Hiccup before being placed on the ground. After Hiccup disappears in the smoke and fire, Astrid rushes to the scene, believing Hiccup is dead. Tears come to her eyes, and when she sees Hiccup again even more tears come to her eyes as she rejoices with the Vikings. After he wakes up and goes outside she sneaks up behind him and punches his back, claiming that was for scaring her. As Hiccup protests angrily, she smiles and pulls him forward for a real kiss on the lips. He quickly tells her he can get used to it as she blushes. She then joins Hiccup and the other teens for a dragon ride on their dragons as the movie ends, concluding their relationship as not always platonic best friends.

It is possible that Astrid had feelings for Hiccup before the events in the movie, due to her immediate change in behavior during the first flight. She might have just ignored this since Hiccup didn't meet the usual Viking standards. When Hiccup was thought to be dead after he defeated the Red Death, Astrid is the only Viking to be brought to tears, meaning she took it the hardest, besides Stoick.

Legend of the Boneknapper Dragon
When Hiccup was upset for Gobber Astrid took a few steps closer to Hiccup perhaps to comfort him. When the they were on the boat Hiccup and Astrid were next to each other and while they were stranded on the island the often is stood next to each other.

Gift of the Night Fury
Astrid remains a close friend of Hiccup in Gift of the Night Fury, as Astrid and Hiccup both advocate the idea to make up new holiday traditions to cheer the tribe up, and they discuss the prosthetic Hiccup makes for Toothless so the Night Fury can fly on his own. He also calls her "M'lady" we she hands him a cup of Yak-nog. It's never officially confirmed if they've become a couple. Even so, near the end of the special, Astrid comforts Hiccup when he's missing Toothless, and even kisses him on the lips and hugs him. She still likes to tease Hiccup however, as she did with him when she noticed Toothless had entered the Great Hall, before she giddily turned him around so he could see Toothless was back.

Dawn of the Dragon Racers
When Snotlout wins Astrid tells Hiccup "One out of two ain't bad, and for what it's worth I always knew red was your color. "Hiccup responds "Aw Astrid thank you." "I was talking to to Toothless." Astrid joked. In the flash back, Hiccup when was trying to get the other riders not to have a dragon race, Astrid was the one he asked to back him up. She did at first, but when he saw the other riders racing including Astrid, he yelled to her, "Really Astrid?!, where's the love for the acting chief?". When Snotlout crashed into Astrid and she fell off of her dragon, Hiccup was worried and upset about her breaking her arm..

Riders of Berk
In the Riders of Berk series they appear to have a budding relationship, but they are quite shy and awkward about it. Their romantic attachment is more suggested in their dynamic and interaction then outright stated. Whilst Astrid maintains more of her initial stern, authoritative demeanor with the other Vikings, she opens up more when alone with Hiccup, and apparently feels safe showing a considerably softer, more whimsical side of her personality to him. The two are incredibly close, with Astrid acting as Hiccup's confidant when he needs advice on personal problems, and often pair off from the others when needing to achieve something especially difficult.

How to Start a Dragon Academy
When Hiccup and Toothless win the best trick contest, Astrid says "He's still the best"! Hiccup was given responsibility of keeping the dragons out of trouble he got so beat up from running from crisis to crisis putting flaming sheep out Astrid went to see him, before she went in his room he said to Toothless "Astrid, perfect I don't look to beat up do I ?" This shows that Hiccup cares what Astrid thinks of him.

Animal House
After Hiccup and Astrid run into an avalanche while riding their dragons, Hiccup attempts to protect her, but they both fall into the huge heap of snow and ice. Their dragons use their firepower to open a hole in the ice so they don't freeze and suddenly realize that they're holding each other, both backing off and blushing.

Later, after Hiccup and Astrid tell Fishlegs what happened, he suggested they could have used each others body heat, to which they scoot away from each other and blush.

Alvin & the Outcasts
In this episode Alvin wants to get the Dragon Conquer aka Hiccup, but Hiccup was hiding until Alvin hurt Astrid then Hiccup instantly showed himself and made a deal with him to leave the people of Berk in peace if he proved he was the Dragon Conquer.

Portrait of Hiccup as a Buff Young Man
Hiccup at the beginning was busy complaining to Gobber and Stoick about his painting as a buff man, Tuffnut asks "Whoa, what happened to Hiccup?" Then Astrid replies "Who cares" in an aroused way.

Later, during the cave-in, Astrid refused to leave Hiccup (who fell into a pitfall) until Fishlegs pointed out that there won't be anybody to save Hiccup if they all got trapped.

Heather Report, Part 1 & Part 2
In Heather Report, Part 2, Hiccup wanted to shadow Astrid on her mission to Outcast Island and wishes her to be safe.

Later, when Alvin grabs Astrid and runs off with her Hiccup gives chase. Hiccup was also ready to kill Alvin when he threatened Astrid's life.

Thawfest
She did say in Thawfest that one thing she "always liked about Hiccup" is that he was a gracious loser, further supporting the theory that Astrid had feelings for Hiccup prior to the first movie.

She is also in a sense Hiccup's moral guide, keeping him honest. At the end of the episode she kisses him for being himself.

When Lightning Strikes
In When Lightning Strikes, Astrid is the first person that Hiccup seeks out before leaving the island, and when Hiccup tells Astrid that he is leaving with Toothless the first thing that she asks if he will come back. Despite his assurance that he will return when the villagers all realize that the lightning wasn't because of Toothless, Astrid appears sad and distraught.

Live and Let Fly
Astrid and Hiccup are comfortable walking closely side by side on their way back from the arena.

Tunnel Vision
When Astrid tells Snotlout that she's sure that's not what Hiccup meant when he started selling water in trade for goods, he says, "Well, that's what makes him Hiccup and me Snotlout." She responds, "That's just the tip of the iceberg.

Fright of Passage
In the beginning when Astrid wants to go after the flightmare, Hiccup tries to stop her. She says "Come on Hiccup don't tell me you haven't been dreaming about the flightmare, going after it, learning about it, training it." Then Hiccup says "Well you know Astrid training dragons isn't the only thing I think about." She then asks him "Are you actually saying that to me with a straight face?" This might suggest that Astrid and Hiccup are thinking about each other more intently. At the end of the episode when the were back in the village Hiccup said to the villagers "We learned alot about the filghtmare and Iwill explain it all to you but the most important thing we learned was Fearless Finn Hofferson was indeed fearless," Then towards Astrid "just like all the Hoffersons." Then he put his hand on Astrid's shoulder and they smiled at each other.

Appetite for Destruction
When Hiccup prepares to leave the Dragon Island to find out what make the islands to sink, Astrid insists to go with him, saying that it could be something dangerous, showing that she cares for his safety. Later, when Hiccup go to confront the Screaming Death, Astrid again insists to go with him and when Hiccup tells her to stay for her safety she ask him "So you're leaving me alone?" even if Fishlegs was here too.

A View to a Skrill, Part 1 & Part 2
These episodes continue the trend of Hiccup and Astrid choosing to be close to each other and the pair showing more concern for each other than typical of the others.

Astrid joins Hiccup, and this time Snoutlout, on the mission to find the overdue Bucket and Mulch, after rescuing the missing Vikings from Berserkers it is Astrid and Hiccup alone who speak to the pair. Once the frozen Skrill is returned to Berk, Astrid and Hiccup stand right next to each other with Snoutlout and Fishlegs on either side of the pair. When Hiccup goes to tell his father about the Skrill Astrid accompanies him and Toothless. While speaking to Silent Sven Astrid stands next to Hiccup, with Snotlout and Fishlegs on the other side. Although, when they land on the seastack after the Berserker attack, they place Fishlegs between their dragons when they land, this could be due to their secondary positions in the V-formation with Fishlegs in the lead. Then when the two teams reunite Hiccup takes up a position right next to Astrid in the formation.

When the Skrill first prepares to fire on Hiccup and Toothless, it is Astrid that cries out his name and rushes forward to help before the others follow. Then when Hiccup and Toothless dodge and the Skrill fires on the group, it is Astrid’s name that Hiccup cries in alarm, despite the fact that the entire groups was fired upon. In contrast, when the Skrill actually hits Snotlout neither Hiccup nor Astrid show much concern, Astrid actually seems to be deriving pleasure from it.

Astrid also acts as Hiccup’s voice of reason and devil’s advocate for his plans. When Hiccup states that he will follow the Skrill into the clouds by using Toothless’ ability to echolocate Astrid asks if that even works in the clouds. Also, when Hiccup places Astrid in charge of the team attacking the boats and he takes Tuff and Ruff to fight the Skrill Astrid points out that he needs to be careful because he has more metal than the others.

Frozen
She hugged Hiccup in front of Gobber when he returned from rescuing Johann in Frozen

The Eel Effect
Astrid was willing to wait outside in the cold while she was suffering from Eel Pox for Hiccup to come back and was very happy to see him return in The Eel Effect. These actions suggest that her crush on him is getting harder to hide.

Cast Out, Part 1 & Part 2
He saved her from falling and even suspends Snotlout after he almost killed Astrid showing that he cares for her safety.

Race to the Edge
Despite there being no direct confirmation, in Race to the Edge, that Hiccup and Astrid are a couple, their relationship is shown to be closer and stronger. It is almost like they both have feelings with each other, and both know the other has feelings for the other, but both feel that there are things more important than a relationship at the moment. However, when they are alone together or one of them is in great danger, they aren't afraid to show a little affection.

Dragon Eye of the Beholder, Part 1 & Part 2
Hiccup appeared upset that Astrid mentioned that she's done searching for new dragons and joining the Berk guard, despite saying how happy he was for her. Astrid then advised him to start thinking about what's next for him and Toothless.

Hiccup also dives down to save Astrid after she falls into giant eel infested waters.

Then while Hiccup is exploring the Reaper, the Dragon Riders are captured by Dagur and locked in a cage on the Reaper. When Hiccup meets Dagur again after three years, Astrid is one he cries out for.

Imperfect Harmony
After they land on Melody Island, Hiccup and Astrid spend most of the time together even slept next to each other. When the Death Song was about to shoot at Hiccup, Astrid pushes him away and the amber immobilizes her.

When Darkness Falls
In When Darkness Falls there are several instances which point to the complex relationship that has developed between the two to this point and establishes a theme that carries through the entire series. When they first land on Dragon's Edge and Hiccup takes charge of having them set up the camp Snotlout complains about liking "whiney Berk Hiccup" more than "Princess outpost Hiccup" to which Astrid replies, "I know, right?" before giving Hiccup a secret double thumbs up. Then the next day when everyone is pushing their ideas for the outpost Astrid smiles when Hiccup states that they need one idea, as if she fully expects Hiccup to choose her design and then gets upset when Hiccup states that everyone else's ideas are good. However, what is perhaps the most telling is the layout that Hiccup chooses for Dragon's Edge. Hiccup placed his hut right next to Astrid's hut and they are closer together than any of the other huts.

Reign of Fireworms
When Astrid is "locked up" Hiccup ask her "So, what are you in for?" to which she replies "Not funny!" and when Hiccup is "locked up" she ask him the same thing. That means that they like to make fun about each other.

Have Dragon Will Travel, Part 1 & Part 2
In Have Dragon Will Travel, Part 2, Heather asks Astrid if she and Hiccup are a couple. First, Astrid panics and throws her axe into the top of a tree. Then she adamantly states the two are just friends, though appears a little nervous on the matter, and it is clear that Heather doesn't buy her denials. Also, when Heather states that Hiccup is kind of cute Astrid blushes, smiles and says, "I guess. If you like that unassuming, heroic, dragon rider type." Then at the end, Hiccup feels sorry for Astrid losing a friend, knowing the two got really close. Astrid then puts her arm around Hiccup and states, "But I still have you."

The Next Big Sting
I'm sure that The Next Big Sting is not often thought of as a Hiccstrid relationship story. However, as a married person, I was struck by how Astrid interacts with Hiccup as he demonstrates his flight suit "Dragon Fly 1" to the gang. Initially, she seems perplexed by Hiccup's fascination with his new "toy". "Ugh, Boys!" she says, shaking her head and doing a facepalm at his delight in tempting fate by jumping off of a cliff to test it. But each time he puts himself in danger doing this, she appears more and more worried about his safety. And whe he finally crashes and breaks the suit, her first words to him are an angry "OK that's it, you're done with this!" And later, when he hints at having an idea of how to mimic the Speed Stingers' webbing in an improved flight suit, Astrid goes ballistic and says "If you say Dragon Fly 2, I'm locking you up!" Granted, this kind of seemingly angry and controlling behavior is not the usual romantic Hiccstrid interplay. But that's the point. Her interest in Hiicup is maturing past the merely romantic. She has invested too much time and effort in keeping him safe to just let him throw his life away on something seemingly trivial in value. Thus, this episode illustrates a silent statement of "I love you, you idiot! Don't scare me like this!"

Astrid's Team
After an attack on Berk, Astrid finds out that her parents house got destroyed during the attack. Fortunately, her parents were out when the attack was taking place. Astrid tells Hiccup that she will be staying in the arena and Hiccup said that he's staying there too, due to his dad's loud snoring but he was really trying to support her. When she decided to stay on Berk to train new riders Hiccup stayed too, he said it was to patrol the island but he really stayed to be on Berk with Astrid.

Night of the Hunters, Part 1 & Part 2
At the beginning of the episode, Hiccup is worried that Astrid has been gone all day without telling anyone where she's gone. Feeling that something's wrong, he decides to lead the others on a search party to find her. Hiccup sees her as she begins to sink into the ocean and immediately dives to save her. As he pulls her out of the water he shouts her name, fearing the worst. After a few seconds she weakly regains consciousness and asks "What took you so long?" Hiccup then holds her in a tight hug thanking the gods she's alright, his panic still evident.

Snow Way Out
Somewhere in the middle of the episode, Astrid tells Heather that she is worried about not telling Hiccup, explaining to Heather that she knew Hiccup trusted her. While Heather reassures Astrid that this is fine, Astrid still looks doubtful. A little after this event, Hiccup tells Astrid that he relies on her. Astrid looks taken aback by that, remembering her talk with Heather. At the end, Hiccup is upset when Astrid tells him that Heather was helping them, since he is surprised that Astrid was keeping a secret from him that could help them majorly. This is also because Hiccup entrusts everything to Astrid, even things that he doesn't tell his dad, and this was a huge act of betrayal. Astrid was scared during this account, because Hiccup was being unusually cold to her. At the end, it seems like they reconcile.

Edge of Disaster, Part 1 & 2
When Hiccup prepares to leave, Astrid implore him to not leave her alone but in the end she accepts to stay on the Edge. Later in the episode she says "Hiccup, where are you when I need you?". When Hiccup finally arrives, she is so happy that she even punches Dagur when he said he hated Hiccup.

Maces and Talons, Part 1 & 2
When Astrid is worried about Heather's safety while working with the Hunters Hiccup reasures her that she would be fine and when he firsts tells her in the stable they look longingly into each others eyes until Snotlout and Tuffnut destroy the moment.

Enemy of My Enemy
At the beginning, she confronted him because he wanted to go on a patrol alone saying that is too dangerous and trying to convince him that he won't find Viggo. She was worried that he would ware himself into the ground looking for Viggo. After she realizes that she can't change his mind, she said "Okay, if you're going, at least let me and Stormfly go with you." to which he replies "Fine!". However he left without her when she went to get Stormfly.

Buffalord Soldier
In the episode Buffalord Soldier, Astrid became deadly ill with a disease called "The Scourge of Odin" and even before he knew she was sick, Hiccup was very concerned, but much worse when Stormfly was acting panicked. He paced around the floor as Fishlegs searched for a cure, and caught Astrid before she fell to the ground, concern etched in his face. While bringing her to the Buffalord she fell off Stormfly he caught  her and shushed her softly, and he told her they will beat the illness and that she must  keep fighting and hold on, confessing he couln't imagine a world without her in it (the most romantic thing he's said so far in the show) before starting to panic when the clear Buffalord saliva did not cure her. He held her hand, and begged her to stay alive a little longer, for him. When Viggo arrived to take the dragon he revealed that it was his boat ware Astrid caught the scourge which made Hiccup's hatred for Viggo doubled especially when Viggo threatened to kill the dragon. Hiccup begged him to let them have the cure and Viggo could have the Buffalord, claiming he would not leave without it, showing he will do anything to save Astrid's life. Finally getting the green solution, he urged her to drink, before helping her up as color came to her face and when she tried to stand up by herself she lost her Balance and she was nearly to fall on the ground but Hiccup was her bond and he caught her. After the Viking funeral for the fishing boat where she got sick, Astrid thanks him for saving her life and she added that she owed him, which he tells her that is not a big deal and he was sure she would have done the same for him, she too confesses she couldn't imagine a world without him in it , the both of them smiled at each other.

A Grim Retreat
Astrid is the one who convinced Hiccup to go on an island to relax. At the campfire, they stay together. When Toothless and Hiccup go into the water she says "come on, come on" showing she cares for his safety.

To Heather or Not to Heather
When Astrid and Heather are talking, and Heather mentions Hiccup, Astrid smiles and looks to the ground. Heather tells her to to express her feelings and that the two of them are perfect.

Stryke Out
When Snotlout accidentally gets Hiccup and Toothless captured and taken to dragon fights Astrid was very worried and when she found him she ran to him and hugged him while they were flying out she said she was glad to have him back and she was worried, Snotlout said he wasn't worried Fishlegs said that Astrid would've killed him if they hadn't found Hiccup.

Tone Death
When Snotlout states that Hiccup can "have" Astrid. Hiccup laughs and says, "Thanks, I appreciate that".

Last Auction Heroes
When the riders were captured, Astrid puts her hand on Hiccup's shoulder and he looked at her and put his hand on her's. Later in the episode, when Hiccup was drowning, Astrid and Stormfly caught him.

Defenders of the Wing, Part 1
When Hiccup tried to train the Speed Stingers, Astrid tried to help him but was stopped when Mala blocked her path with her sword and then Throk held her back.

Midnight Scrum
When they are alone in the Great Hall she puts her hand on his hand and tells him to inform Stoick about the bounty. After Astrid and the other riders find Hiccup she immediately rushes over to him and helps him stand without his metal leg.

Dire Straits
After dragging Hiccup on deck Asrtid calls his name, feeling worried that he isn't breathing. When he finally wakes up she is relieved to see he's ok and puts her hand on his shoulder. It shows how much she cares for Hiccup. Kinda like what Hiccup does in the episode Night of the Hunters Part 2.

Out of the Frying Pan
Just before she passes out while in the lava pit she mutters Hiccup's name and when Hiccup, Fishlegs and Mala spot Astrid and Throk Hiccup lifts Astrid onto his lap.

Blindsided
After Astrid had gotten blinded and knocked over by a flash of lightning, Hiccup is concerned. He is even more concerned once he learns that she is blind. Hiccup later grabs Astrid's waist, and pulls her back onto Toothless telling her he was glad she was there as she apologised and then proceeded to hug him. When they land, Astrid begins walking off to find Stormfly when she trips over a rock and Hiccup leaps for her wrapping an arm around her waist and shielding her head from hitting the ground as they fall, leaving them in an awkward situation. When the Triple Stryke first appears, Hiccup pulls her out of the way and shortly after has Toothless grab her and move her to a safer position. Later, Hiccup tells Astrid that Stormfly was only protecting when she was cast of to the side by her dragon. Hiccup cheers her up and tells Astrid that he is there for her, that he will always be there for her, that there will always be a Hiccup and Astrid, and that she is far from useless in her current state. Shortly after Hiccup says this the Triple Styke returns and Hiccup dives for Astrid pulling her to the ground with him and out of harm. Later, Hiccup and Astrid are sitting by the Dragon stables, when all of a sudden Hiccup moves as close as possible to Astrid. Astrid then asks Hiccup why he did not kiss her when the opportunity arose. Hiccup states that he was waiting for the perfect moment to do so. Astrid replies that right now is the perfect moment, and then Astrid and Hiccup share a kiss, and with it their romantic relationship is started.

Shell Shocked, Part 1
Astrid congratulates Hiccup for creating the perfect plan to capture and defeat Viggo, but while Hiccup is still nervous and unsure, Astrid assures him. They attempt to share a kiss until they aren't interrupted by Fishlegs, but he instead believes they're thinking of ways to hide the Dragon Eye once regaining it, completely oblivious of their romance. Hiccup and Astrid are relieved and decide for the time being to keep their newly formed romantic relationship a secret from the rest of the Dragon Riders. Eventually, the rest of the Dragon Riders being to notice a change in Astrid and Hiccup, but all are oblivious of their newly developed romantic relationship and instead theorize different reasons for their behavior. While Ruffnut and Tuffnut believe that Hiccup is going to die and Snotlout believes he has developed a gambling problem, only Heather becomes aware of their romance and allows the couple to have time alone together.

Shell Shocked, Part 2
Viggo knows of Hiccup and Astrid's relationship, and taunts him with it, telling him how odd it is that he is in love with a girl who is also with him in battle, which winds Hiccup up, forcing him to concentrate on the task at hand, stopping Ryker.

After defeating Ryker Hiccup goes to check on Fishlegs, after Fishlegs leaves Viggo appears while holding Astrid captive and states that he will release her in return for the dragon eye, causing Hiccup to throw it into the volcano which makes Viggo let go of Astrid and run to it, this show how Hiccup will do whatever it takes to protect Astrid. At the end of the episode, just prior to the explosion of the volcano on the Edge, Hiccup and Astrid were kissing after promising to keep their relationship out of the war with Viggo, but they were seen by the Riders, erasing Ruff and Tuff's suspicions about an 'illness'. Snotlout, on the other hand, seems to be convinced of a gambling problem.

How to Train Your Dragon 2
By the time of the sequel, Astrid and Hiccup are an official couple, boyfriend/girlfriend or possibly betrothed. She even teases him in an affectionate manner, and calls him babe when they are fighting Drago's army. They are comfortable sitting very close to each other, as opposed to in Animal House when they nervously moved away from each other. They are even so close that Astrid casually braids Hiccup's hair. Astrid also helps Hiccup in personal problems he can't figure out on his own. Also, she has displayed greater loyalty, trust and protectiveness towards Hiccup, just like Valka throughout the movie. They exchange two cheek kisses and a big kiss on the lips at the end.

Deleted Scenes
How to Train Your Dragon: An Axe to Grind

An Axe to Grind was deleted because of the length, not the content. It's therefore considered canon.

This deleted scene gives insight into Hiccup and Astrid's relationship prior to dragon training. It also shows Gobber encouraging and teasing Hiccup about his crush on Astrid.

Near the end of the scene, Astrid thanks Hiccup for sharpening her axe and then brushes her bangs away from her face with a smile. She does the exact same thing before kissing him in the cove and after almost kissing him in Animal House. This suggests that there might have been some aspects about Hiccup that already appealed to Astrid.

Gift of the Night Fury: Missing Toe

Trivia

 * Hiccup and Astrid relationship is usually shortened to "Hiccstrid" when referred to in a romantic context. It is mentioned on-screen for the first time in Race to the Edge: Season 4 episode Shell Shocked, Part 1 by Snotlout.
 * In the song, "Fly Dragons Fly," which aired on Cartoon Network, a part in the lyrics said, "Astrid and Hiccup, each other they like. Wonder if they marry? They might."
 * In the 2nd movie this is the first time Hiccup kissed Astrid instead of the other way around.
 * Astrid and Hiccup have kissed on the lips 7 times.
 * They are the only ones throughout the franchise who kiss besides Stoick and Valka.
 * As of How to Train Your Dragon 2 Hiccup and Astrid are the only Riders who are dating.
 * Hiccup liked Astrid before Astrid liked him.
 * Astrid always screams "Hiccup!" when something happens to him and Hiccup always screams "Astrid!" when she in danger.
 * Snotlout tells Hiccup that he can "have" Astrid because he likes Heather. Hiccup laughs and thanks him, acting like he already KNOWS who Astrid has chosen.
 * At the end of HTTYD 2 just as Astrid and Stormfly are about to catch the black sheep Hiccup and Toothless grab it in at the last few seconds and puts it in Astrid's sheep pile instead of his own. This is similar to what Fishlegs and Snotlout did for Ruffnut at the beginning of the movie
 * Their relationship officially starts to take form in Race to the Edge: Season 4 in the episode "Blindsided", and it was intended to stay secret, however, they were seen kissing by the gang at the end of "Shell Shocked, Part 2"
 * According to an upcoming interview with Richard Hamilton; "Hiccup and Astrid lead Berk as a pair by working together as chief and general."

Quotes
The page for quotes about Astrid and Hiccup's Relationship is here