this feels like it's supposed to be a big revelation, like why is such an important person looking after some random kid who somehow awakens to immensely strong powers?
it could mean that akutsu might be more important as a person than we've been led to believe so far and that yamada had to test/keep track of his progress of learning how to use his strength as well as recognizing amane from somewhere(chaper 40, but we've been led to believe it was just another gag). That being said, yamada probably already knows she's an angel and resembles someone he used to associate with who may not be alive anymore.
also most importantly, his name may be Sebastian after all, the biggest bamboozle as of now! How could they do this to us?