Sirius loves "Rebel Girl", so he and Marlene are obviously BFFs.
How do Sirius' politics (or lack of specific ones) manifest? Or is it straight out and plain rebellion against his family? Does he believe it on his own?
Just curious, since that's a big part of Marlene's life and it might affect their interaction.
Oh, and they're quidditch teammates. Go go Gryffindor and all that.