Astrid and Hiccup's Relationship

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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. 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 - due to her thinking he is not like the other Vikings. However, whilst the other Vikings actively ridicule Hiccup, Astrid seems more suspicious towards him, only becoming angry with him due to his incompetence in the Arena. Her dislike for him grows into jealously when he becomes more skilled at 'fighting' dragons than she is. Despite the initial problems in their relationship, Astrid later grows to where Hiccup becomes her main love interest as she kisses him twice (albeit after punching him on the arm both times).

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.

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. 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.

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.

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.

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.

A View to a Skrill Part I & A View to a Skrill Part II
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.

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.

Dragon Eye of the Beholder Part I & Dragon Eye of the Beholder Part II
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 her cries out for.

When Darkness Falls
In When Darkness Falls there are several instances which point to the complex relationship that has develop 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.

Have Dragon Will Travel Part I & Have Dragon Will Travel Part II
In Have Dragon Will Travel Part II, 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 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
The Next Big Sting

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 even exchange a few cheek kisses and a kiss on the lips at the end.