Hmmm

Maybe he's just hand in his resignation letter like normal salaryman quit his job. (〜￣△￣)〜

Hmmm....The logic. I understand

Please tell me that's actually how it works, I'm laughing so hard.

Falling in love with a human. At least that is the folklore. Incubi and Succubi are heartless. They "fuck and truck" and are only in it for themselves - in fact, they are only in it because they derive life force from sex. Basically, they eat and run. It requires a heart to feel love (again, folklore not fact) so, if an incubi or succubi "grow a heart" they become human. That is why the "ex incubus" said, "You only have a limited time." Demons are immortal, humans, obviously are not. He knew the younger demon was no longer an incubus because he had grown a heart and fallen in love. Just as he had.

cool! Good to know

But he just turned into a bat ten minutes before...? Humans definitely can't do that, so, maybe not *yet* but he'll turn human soon?

Perhaps. Since this is a manga, the writer can twist folklore in any way they choose. Artistic license and all that. It's the same with vampires. According to folklore, they don't "sparkle", but a reflective skin would account for not being seen in mirrors or on film. A little twist here and an embellishment there and you have a whole new interesting story.