Headcanon-ing Sirius as rebellious and loud is okay. But just think about Sirius who is this trained and polished boy. He was raised in an aristrocatic family, with tutors and violins. He talks like royalty, he walks like royalty, he eats like royalty.
It would take him atleast an year to stop from judging peopla based on the way they eat or the way they dress. He hates himself for this. He can't stop. He won't be as loud and as rebellious till atleast 4th year. He is sharp, he knows how to read moods based on your eyes or tone (thanks to walgurba black's A+ parenting), he never shares anything because god knows the other person might use it in next argument (again thanks to walburga).
Sirius black who catches himself about to say something conservative at a conservation. Sirius Black who never slouches while sitting. Sirius Black who figures out Remus' secret within an year because he keeps note of his disappearances.
Also Sirius Black who tries to be different from his family, but also wants to be a 'perfect' Black to gain his mother's affection just once.
I love this! We as a fandom need to use Sirius’s ex-cult persona more often, because any kid who grows up in a family like that has some views that are hard to break out of
