Cute facts might be a bit difficult, thank the way his series unfolded. Nonetheless, let's give it a try-
-Supposedly, his favorite food is spaghetti. This may be a tad dubious, given the source and context.
-Despite some of his words and actions, he's quite receptive to physical affection. He likes hugs and even initiates one himself at the end of the manga.
-Evidently he's quite a wordsmith, with ostensibly no prior experience he wrote a poem that immediately plunges the reader into a deep depression. One wonders what exactly he wrote.
-He named the corner of his room "Mr. Corner", and tends to spend a lot of time sitting in it. According to him, it's calming.
-He doesn't appear to know how to shake hands at first; when offered a handshake by one of his friends, he tentatively taps her hand instead. It's cute.>>3257948
Classical, maybe? In some ways the baroque elements fit him. Can't imagine he's had much exposure to music in general, so my preferences for 80's pop et al might rub off on him.>>3258009
All kinds of things really, even aside from the most obvious one. Since it's more pertinent now with the changing weather, it'd be nice to know if the fan theory of him having a lower body temperature due to his black blood is true. It seems sound but it's hard to say, given some of what's been said about it in series. I'd also like to see more of how he'd fare in normal everyday situations. A relatively brief peek of it is given, but a lot changes thereafter especially in the manga. I could go on and on.