Archive of Our Own

Summary:

Atsuko Kagari has always dreamed of becoming a legendary Viking and dragon slayer like her idol, Shiny Chariot.
But when finally given the chance to do just that, she can't seem to bring herself to kill the mysterious Night Fury she shot down. So instead, of course, she feeds it, befriends it, and teaches it how to fly again.
It turns out that they might have been wrong about dragons.

or

Akko goes against all she knows to befriend a dragon in secret while Diana gets increasingly suspicious, Ursula becomes increasingly suspicious, and Akko's friends get more exhausted by the day.