Character headcanon/theories
Arthur Weasley – pretends to be affable and silly to be underestimated and get more information than he would otherwise; corrupt
Severus Snape – killed James Potters parents with Sectumsempra during DE time, Snape was their accountant (or someone was at least)
Flamels – never died, had backups of the stone (Dumbles told Harry otherwise to encourage the idea of death = next great adventure); the stone was either fake and they just had Horcruxes or the Stone requires a death toll as well (law of equivalent exchange and all that)
Molly Weasley – kept Arthur under a love potion -> Bill conceived under the influence of one -> Bill should be ruthless and such
Ginny Weasley – horcrux or parselmouth thanks to the diary
Kreacher - was such a dickhead because nobody ever bothered to treat his exposure to bottled dementor and so he's been suffering as though he were an azkaban prisoner for more than a decade
Sirius Black - had the Black madness after all, which is why he pulled the Snape-murder-stunt and was such a general shit head
Peter Pettigrew - is actually a very valued and high-ranking Death Eater, since he brought Voldi back AND has a decade of spy intel on the Weasleys (aka family with close ties to his two biggest enemies)
Luna Lovegood - tortures while/with singing nursery rhymes