@soulffles Oh, Im glad to answer it! (Just needed some time to prepare, hah). Part 1 just in caseTime for dumb headcanons:
Firstly, I think he would be pure-blood, but, of course, its not a strong reason. Agreste family surely wasnt the Death Eaters, hah, but also wasnt supporters of their ideas, even being the most of them in the Slytherin House (maybe his mom would be in the Ravenclaw, for example).
Secondly, yeah, I guess, the most of us think than Adrien is more perfect for Hufflepuff house because of being the biggest cutie pie. But, whats funny, maybe because of this he also could be in Slytherin. How?
Too simple, huh? Well, Im not a big psychologist.
Actually, some mages and witches were strangely chosen in the Houses in canon (the best example is Peter Pettigrew)
That was the most normal reasons
Thirdly…. Well, green color suits him, lmao. (not sorry)
Fourthly, for me, it just makes interesting plot and also interactions with other characters
With Nino, for example
((Sometimes they make stupid jokes about it))
Okay, i can talk about Hogwarts AU really long time, so i need to stop. I hope, theres enough for explaining about Adrien being in the Slytherin
(i know, theres not enought and my explanation is stupid) OKAY HE IS JUST LIKE SCORPIUS AND THATS ALL