my idea is: Der is a sphinx that has been spotted in the modern word and thus an "Anomaly"('specially since he's humanoid)
So like kinda SCP foundation
Avery probably is the one to catch him,
Avery "catches" him, but knows a foundation would run tests to either eliminate him OR neutralize him
(I think when Avery "catches" him, he just assumes it is catbirb, because when he picks him up he isn't humanoid at all, he just has glowy eyes and looks like a cat with wings)(Der is too tired to fight when Avery finds him so just lets himself be kidnapped, besides, this human isn't the worst,,, soooooooooooo)
Der ends up going right back to the humanoid form the moment he has the energy to, wants to leave, but doesn't
Avery finds out sphinxes do the riddle thing... and that it's an enrichment thing. He also finds that the sphinx he brought into his house is more polite than most houseguests and that the one he's got is probably the last one(THERE USED TO BE OTHERS???)
(He learns sphinxes aren't immortal, and that they are solitary with their own species, only interacting for mating and courtship, but that they are monogamous) (And apparently they can tell how close the nearest like them is, aka other sphinxes, and that Der knows he's the last because of that)
Der likes Avery and is kinda like "My human" after a while
Avery is content with the catbirb of knowledge, it lives in his house and reminds him when he's got important stuff to do. It's also fluffy and it purrs. His opinion is that the only bad thing about the catbirb of knowledge is that it nests in his attic and gets a little bit territorial over it sometimes
@tstdbreadwgarlic You're my fellow slimeknight moot, what do you think?