Astrid and Stormfly's Relationship

Astrid and Stormfly have a very close bond with each other.

How to Train Your Dragon
When Astrid and Stormfly first met in the Dragon Fighting Arena, it was during an exercise. After attacking the other teens with her spines and fire, the Deadly Nadder chased Astrid through the whole maze, destroying everything. The fight finished when Astrid hit the dragon with her axe and a shield.

During another training, Astrid fought Stormfly again, but before killing her, Hiccup scratched her under the chin, making her fall asleep.

Later, Hiccup helped everyone train a dragon, and so Astrid and Stormfly bonded. They later helped Hiccup, Toothless, and the other teens in their battle against the Red Death.

Even though Astrid fell off her dragon during the battle, she was later seen riding Stormfly during Hiccup's closing narration.

Gift of the Night Fury
Astrid and Stormfly were seen decorationg Berk together. Stormfly is following Astrid's commands, hinting that they have trained a lot since the battle with the Red Death.

However, not even their strong bond could stay in the way of Stormfly leaving the island. It was later revealed that the dragons went to lay their eggs.

When the dragons return, Astrid and Stormfly reunite, with the latter also bringing her babies. She even lets Astrid touch them. They then celebrate Snoggletog in the Great Hall.

Dawn of the Dragon Racers
The duo seems to have a strong bond, both in present time and in the flashback.

Dragons: Riders of Berk
In "How to Start a Dragon Academy", Astrid and Stormfly are revealed to have trained more, as Stormfly knows a lot of tricks. It is also revealed that every morning, Stormfly is the first thing Astrid sees in the morning.