https://www.youtube.com/watch?v=MWVzVr9t2gU

Kass should have been the one to fight the Legendary Beast
with you after you hear all his songs and he returns to the
village, that should have been what kick-started the battle.

Just imagine if his remix from Smash Ultimate played while he
worked with you to board the Divine Beast instead of that other
bird character.

Kass was so much more impactful and there would have been so
much more meaning if he helped you. It could have been his way
to atone for his teacher's envy/jealousy toward Link 100 years ago.

"My teacher... He had... Feelings... Toward you. But I... No... I will
help you in the ways he couldn't. For my teacher, I will atone for
his spite and cowardice. Though I do respect him, there is much
to atone for, and I will carry you upon my wings as I carry his sins.
Together, perhaps atonement can be found through this fight..."

"Come now Link! With your courage and my fervor, we will create
a ballad none shall soon forget in the skies of battle!"