I'm thinking about concepts of other monsters of Byleth
(I'm saving dragon for the post obviously.)
But tell me why the concept of Incubus! Byleth is so cute.
He has never had an actual meal, He's only ever been feeding off of lustful thoughts and other scraps of lust his whole life.
And here comes this human who brings him home from the bar because they kissed him once, and he wants more, or maybe he's too polite to break away and say no, or maybe he's just curious to see what all the fuss is about from his demon friends.
His first taste of a human, and suddenly his disguise breaks. Poor thing looks like he's been caught with his hand in the cookie jar. His wings and tail are slightly between his legs as if he's trying to look smaller.
Byleth is an incubus, a demon known for their suave charisma and you know what he says?
"I'll make you feel good... It'll be worth your while."
Of course in the most deadpan toneless voice you have ever heard.