I have such weirdly specific headcanons...I scare myself.
Okay, but a Sirius who is completely and totally into 80s heavy metal. Why? Dunno, just is. And and and...as a teenager he used to sneak out of Grimmauld Place (and by sneaking out I mean slamming doors and revving that earth shatteringly loud bike of his) and driving to London to watch bands perform.
The energy and rebellion out there is such an exhilirating release from his life at home.
Also, for a brief period (before the War broke out and life was still semi normal), he was the lead singer for a Ronnie James Dio cover band. His cover of 'Last in Line' was exceptional and who knew Sirius could sing like that?
And him replacing the bikini women pictures in his room with photographs of him chilling with his Muggle band, up on stage, at crazy parties and of course, it drives Walburga crazy because it's proof that not only is he a shameless blood traitor but he's never, ever going to be anything else. It's just not in him and she hates it so much she can't stand to look at him.
I see this more than anything else as the catalyst for him finally leaving Grimmauld Place for good.
And when he was locked up in Azkaban, trying not to give in to the overwhelming urge to curl up and die, his fingers would strum at a guitar that isn't there and an old song would come back to him, and just for a minute, he would see himself back in those dank, dingy clubs singing 'Last in Line' with a vicious glee that reminds him of who he is and why he's here and why he needs to hold on.
And when the war is over, and Harry is finally safe and he knows his best friends- all of them- are gone but they can finally rest, he feels a sense of peace, but also a deep loss.
Because he really is the Last in Line, there's nobody left. He has nobody and nothing left.
Nobody but him. And his bike. And a rusty old guitar that he found stashed away in the attic of Grimmauld Place.
And somehow that's enough, he thinks as he sits by a window and plucks at the strings and smiles. It was always enough.