This is something I think about frequently, and I needed to put my thoughts down.

Most people I think, off hand, would say Slytherin, what with being a demon and evil and all. And I’m here to tell you why he’s not.

Slytherins are ambitious and cunning. Evil is often attached- no. And while Sebastian is a sly and cunning motherfucker, it's usually due to wordplay and getting around an order from Ciel. But he never seems particularly ambitious; nor does he ever show a yearning for power. (except maybe over Ciel to fuck with him.)

Hence, he’s not a Slytherin. 

The next house is Ravenclaw. A house for the wise. And while Sebastian is insightful, he isn’t particularly wise. Yana Toboso herself has said he’s the brawny over brains type. Sebastian also doesn’t show a particular thirst to attain knowledge either. He doesn’t value knowledge unless it can help him with a contract. 

So, not a Ravenclaw.

The most far fetched one, Hufflepuff. Despite being the most farfetched, I actually think it, at second glance, might be the one people guess. Hardworking and Loyal. That literally describes Sebastian to Ciel. Except. It’s for the contract. Sure, he’s hardworking for his meal- he’s putting in the time and effort. But Sebastian himself has said even if it takes years, thats like the blink of an eye or something(im paraphrasing.) So he’s not all that hardworking. And when he thought the meal wouldn’t be worth the patience (after Ciel received his earldom, and when he had the mental breakdown during the witch arc) he was going to just eat Ciel, and not even try. Regarding loyalty- if not for that contract, Sebastian would hold no loyalty to Ciel whatsover. Sebastian’s first loyalty, after Ciel and Ciel’s image, is himself.

So he doesn’t quite fit that frame.

Now, finally, Gryffindor. First, as I said earlier, Yana has said he’s the brawny type. He’ll sooner rush into things, (like eating Ciel) then take a moment to figure out the who what why. Regarding character traits: Daring, Brave, bold- while they don’t necessarily fit him correctly, they do coorelate with his actions. By all appearances sake, he does seem brave (but what is bravery with an absence of fear??) So, it’s a lopsided canvas- much like how hufflepuff can fit by appearances sake. However, one particular trait of gryffindors (that crosses over with slytherin) is arrogance. And its not the slytherin way of thinking (”i’m better than you because i have money/great aspirations/etc”) its?? “I’m better than you because I genuinely think that. Because I’m stronger, I’m superier, I’m a better person, etc.”

And Sebastian is arrogant. To a fault. He believes in his abilities, and very rarely does he take an opponent seriously. (the only one I can actually think of are the shinigami.) And despite shinigami being able to seriously wound him- He still doesn’t doubt his abilities. The only one he hesitates with is Undertaker- and thats only after he learns the hard way “fuck this isn’t going the way its supposed to.” 

And this makes that canvas fit a little bit better in the gryffindor frame.