OH BOY I think this CR could be completely insane, I'm so ready. Totally canonblind to your guy, but mine is a fragment of Hastur, a Cthulhu-tier eldritch god from realms beyond reality etc. An adventuring party fucked up his summoning ritual, so he lost his memory and became bound to a human host. By the time he remembered he's a god intent on cracking the world like an egg, his host won him over with ~the power of friendship~, so now "John Doe" is all in on living as a mortal. Would you like a lecture about how humans are great actually? No?

Apophis will probably remind him of his own sadistic chaos villain, so the negative CR will be mutual. If John recognizes the fumbling attempts to care for humans, he'll become grudgingly fond despite himself.