(I’m assuming you mean for Kai? *hopes this is correct since mun doesn’t know HP so well to have AU headcanons for just HP*
He’d definitely be a Hufflepuff, though I don’t think he’d be out of place in Ravenclaw or Gryffindor. Like the twins, he’d be a prankster, using both the mundane magic and the real stuff. One of those headcanons people have for muggle-born students AUs where they mess with people by using sleight of hand? Yeah, that’d be him. Though, Toichi would also be/have been a wizard, so he wouldn’t be from a completely magic-free family. (Much as I like Chikage, I imagine her being the type who doesn’t have magic herself, but accepts it without much question.)
Kai would try and make people think he could care less about studying, but secretly studies magic above the level he’s supposed to be learning. He wouldn’t like to misuse it, though, so would still prefer his usual type of tricks. Real magic can go wrong too badly for him to use it carelessly. The only person who knows he learns more advanced spells than he should is the Charms professor, because he noticed Kai use one he shouldn’t know, even though Kaito himself tried to play it off as one of his usual other tricks.
Before this gets too long, other random things are: He has friends in all of the other houses (not too surprising), he’s explored the castle quite a bit and knows where almost all secret passages are even without the Marauder’s map, he uses doves to deliver letters instead of an owl (yes, more than one, for if he gets any heavy packages and they’re usually with him, except at mail time.), and other subjects he excels in are transfiguration, flying, and history of magic.)