Just saw your xB drawing and I NEED to know all of your headcanons for him NOW please and thank you <3
(I am being serious, I'd love to hear you yap about him)
I'll try not to yap too much but no promises.
My main headcanon and a big aspect to my design for him is that he's part guardian, as many people headcanon, but I like to think of him as one who had passed the role of elder to become an ancient guardian.
As a result, his fins and tail had began to decompose. It's completely harmless to xB though his exposed bones are especially sensitive to heat. His vision had become enhanced with this transformation, causing him to need a helmet or some kind of eye protection that can help protect it from light and heat. (He'll wear contacts that darken his vision sometimes)
More recently he had began exploring outside of his self-proclaimed territory, and he had years worth of materials that he had collected so he'll sometimes offer them to who he sees as children (the other hermits). He is more giving to those he thinks needs it more, and he'll take from others just for the fun of it.
Keralis had been helping him adjust to the more mechanical side of things, going so far as to create his helmet and wings for him. Keralis decided to give it a dragonfly design after nicknaming xB 'blue dasher'.
This is all mostly based on his actions for the most recent season, I unfortunately only started recently watching, but I'll probably have more headcanons the more I watch.