|“||Thanks, Astrid. It was nice having a friend again. [src]||”|
|— Heather to Astrid|
In Heather Report, Part 1, Astrid positively despised Heather with a rather good reason, being very suspicious of her actions. However, after a bit of a rough start, and a couple of misunderstandings in Riders of Berk, Astrid and Heather became the best of friends and in Dragons: Race to the Edge, they occasionally tease each other about boys, specifically about Fishlegs and Hiccup. Heather knows how much Astrid cares about Hiccup and states that the two are "perfect for each other". Astrid is very understanding about what Heather is going through and doesn't hesitate to be her friend, and Heather is loyal to Astrid in many ways the both of them can relate to.
And just like any friendship, the two go through some problems of their own but with each other, they overcome those challenges, like any best friends should.
In Riders of Berk, Astrid and Heather did not get along well at first as Astrid had evidence that Heather was a spy for the Outcasts and believed that she had bad intentions. She was also angry that Heather had apparently convinced Hiccup she was harmless and that Astrid was crazy. However, they made up in the end as Astrid had discovered that Heather's true intention was to rescue her parents from the Outcasts. Astrid then turned to say goodbye to Heather in the end scene, as Heather set sail back home. Astrid was pleasantly surprised, as Heather gave a sudden hug, with the words: "Thanks, Astrid. Thanks for everything."
Astrid was deeply impressed by Heather's change, unlike Hiccup, and was an instant fan of her self-made double axe. While Hiccup appeared to be skeptical, Astrid loved how intense and edgy she had become over the years; she clearly appreciated the fact that Heather was no longer a sweet, pleasant, pretty little girl, but a hardened warrior like herself. They also bonded over their mutual disdain for Snotlout's obnoxious flirting. It is obvious just by their actions towards each other that they'll soon be close friends.
Astrid pulls Heather out of the clubhouse for some "girl time" to make her feel less overwhelmed. It then becomes obvious that Astrid's lack of female companions of her age and spirit when growing up had taken more of a toll on her than she had realized, and that she enjoyed having a companion after her own heart to talk with. They head to another part of the island and practice their ax throwing. Heather shows Astrid her double-sided ax, and Astrid wants Heather to show her how she made it, impressed by the weapon indeed. Heather tells Astrid she prefers close combat to throw, and Astrid says it's not a bad tactic and then displays the "element of surprise" tactic, which Heather realizes and dodges. Astrid also states that it is one of Hiccup's favorite tactics. Curious and slightly surprised just how much Hiccup and Astrid have grown over the years, Heather asks Astrid if she and Hiccup are a "thing", which causes Astrid to throw her ax wildly, due to the many thoughts running through her head at Heather's sudden question.
Astrid says they are just friends. But Heather doesn't buy it stating that she has seen the way Astrid and Hiccup are together, also carrying a curious smirk, making Astrid more nervous about her words. Astrid repeats that they are just friends. Heather teases her by saying Hiccup is "kind of cute", to which Astrid replies, "I guess. If you like that unassuming, heroic, dragon rider type." Astrid then says Heather and Snotlout are a match made in Valhalla (with a hint of sarcasm), but Heather says she is not into the "macho Viking type" and that she likes smart ones, like Fishlegs. Astrid is surprised at that and Heather says that she thinks he is funny and cute. Heather then says they have had enough boy talk and asks Astrid what else they do for fun around here. Astrid and Heather then run together and jump off the cliff and fall onto their own dragons for fun. They are, at this point clearly becoming friends.
In the end, Heather is preparing to leave the Edge and go figure things out, and Astrid tries to convince her to stay, saying that she is there for her, but Heather does not change her mind and thanks Astrid for being her friend saying, "It was nice having a friend again", showing that even in just a span of one afternoon, Astrid and Heather already have a big effect on each other.
Astrid and Heather appeared to be fighting all the time while Astrid and the riders were being held hostage by Ryker and his crew. Astrid was shocked and slightly hurt by the thought that Heather had betrayed them. It was later revealed to be an act they put on to make Ryker believe Heather was on the hunters' side.
Heather made Astrid promise not to tell anyone else, including Hiccup, about her plan to take out the hunters from the inside, which also caused some problems on both Astrid and Hiccup's side of the relationship.
Astrid and Heather had been meeting up in secret behind Hiccup's back and Astrid appeared to be struggling with keeping secrets from Hiccup and lying to him. Astrid was later forced to reveal Heather's true loyalties when Hiccup was about to shoot her and Ryker down.
Astrid was worried about Heather and wanted to pull her out but Hiccup reassured her that Heather would be fine and she could take care of herself.
Astrid, Hiccup and the riders tried to find a way to save Heather from her impending death. After being set free by her brother Dagur, Heather later appeared in front of Astrid to block the Flightmare's attack. Astrid was very happy to see her friend again even though Heather ended up taking off without a word afterward.
After Heather arrives at the Edge, she shares a hug with Astrid, having not seen her for months. Astrid and Heather then talk about why Heather didn't come back with them after the Viggo fiasco, and Astrid says she was worried about her. Heather than mentions that during her time with the hunters, they were playing a part and that she did not have time to think about Dagur being her brother. Astrid then tells her to not think about it, but Heather responds she has to and why she has to, with Astrid saying that if Heather was anything like Dagur they would have dropped her into a volcano a while ago. Heather then says they should talk about something less depressing, like Hiccup. To which Astrid says, "What about him?" in response. Heather, knowing that Astrid has feelings for Hiccup says, "Astrid, life is too short. You need to express your feelings. Stop wasting time. You two are perfect." Astrid replies that she could say the same about Heather with Fishlegs, adding that the two of them have practically worn out the flock of Terrible Terrors. Heather responds by saying that she has no idea what Astrid is talking about.
At the island where Hiccup is being held, Astrid and Heather fight the Dragon Hunters together, and Heather helps Astrid find and rescue Hiccup.
Astrid and Heather were teamed up with Snotlout for a mission where they had to put up with his flirting again.
Astrid was incredibly protective of Heather when Dagur showed up and promised to make him pay if he ever thought of hurting Heather again. Just like Heather, Astrid initially suspected him to be a spy as well but was also teaming up with Hiccup at the same time.
Astrid and Heather teamed up again to save the other riders from being discovered by the hunters.
While being held captured by Queen Mala, Astrid and Heather tried to plan out an escape where they would be working together to take out the guards.
It's revealed that Heather had moved into Astrid's Hut and they have been living together. Heather is shocked to see how ruffled Astrid's hair is in the morning.
During Berk's 400th anniversary celebration, Astrid and Heather teamed up again to perform a stunt.
Heather gave Astrid flowers as thanks for letting her borrow her axe.
Astrid told Heather to be careful out there with Dagur and was certain she would see her friend again.
Heather is able to figure out that Hiccup and Astrid are in a relationship before the other riders are and is willing to give them time alone, walking away from them with a smile on her face, happy that Astrid has finally admitted her feelings to Hiccup.
After Astrid arrives on Berserker Island with the other riders and is cleared through the checkpoint, she shares a hug with Heather.
Heather asks Ruffnut, Tuffnut, and Hiccup if Astrid and Snotlout will be long, as she wants to say goodbye to Astrid before she heads home to Berserker Island. After Astrid gets on Stormfly and heads back to her hut, Heather says that she'll just go and talk to Astrid herself.
Astrid joined in on Dagur's rescue mission at the end and was happy to know that Heather had finally found closure.
- Astrid and Heather are the only characters to have their friendship listed on their profiles in the official Viking Guide. 
- Heather was based on Astrid's character model and was even planned to be similar to her in personality.
- They look similar enough that Astrid is able to disguise herself as Heather well enough for even people who knew them both to be fooled.
- Heather knew about Astrid's feelings for Hiccup long before anyone else did and Astrid was the first one to learn about Heather's feelings for Fishlegs.