Post canon owlboy content where
Strix refuses to teach Solus any longer, after knowing what the boy caused.
So Asio, feeling responsible for how Otus and Solus have been treated and wanting to grow as a person, offers him to become his student.
So the three of them kinda become a family.
Otus and Solus now have each other to study with and they comfort each other if they feel anxious and Asio tries his best to make up for the years of verbal abuse he put Otus through, in a way learning more from the students than they learn from him.
And eventually, Solus and Otus develop feeling for one another. Asio doesn't know how to deal with teenagers but he does his best. When the two of them get their diploma it's an emotional moment for everyone, even if Asio really knows deep inside that the two pupils could have graduated right after they saved the world, years ago, but he just wanted to keep them around.
Nothing changes much after they graduate though, they still like to hang around their stern yet now kind father figure. And they invite him to their wedding.