Sirius Black is Harry Potter's godfather, which seems to imply that wizard kids are baptized. Kind of surprising, right? Usually in modern times, a Godfather's only real duties are to stand around during the baptism and then perform a terrible Marlon Brando impersonation afterward. The traditional responsibilities of a godfather, however, also include assisting in the religious upbringing of the child (something that doesn't really come up in Potter), protecting the child (something Sirius at least attempts), and raising the child in the event of the parents deaths (well, he wanted to).
Sirius Black is an Animagus, a rare wizard with the ability to transform into one specific animal. Sirius, coincidentally sharing a name with the Dog Star, just so happens to transform into a massive, black dog. Also, his favorite Led Zepplin song is "Black Dog," though maybe that's not much of a coincidence. I mean, it's a pretty sweet song. What do you think, a guy like Sirius Black is going to be into "Stairway to Heaven?" I don't think so. "The Battle of Evermore?" No. "Black Dog." It's awesome.
"Didn't take too long 'fore I found out, what people mean my down and out.
Framed me for murder, perfect plan, sent me away to Azkaban.
I don't know but I been told, Lord Voldemort ain't got no soul."