Funnily enough, I had no idea what Cowboy Bebop was when I first playtested the game and found Ed/Ein. Dave, I'll just second how good it is.
So the only other character I know how to find is Spike. It's at the end of the first scene, when you break into Matthias's apartment, but DON'T look up his phone number yet, as that will end the scene. Instead, once you're inside, immediately leave the apartment (as soon as you can, anyway) and backtrack to the starting screen. Spike will be there.
I had no idea that other Cowboy Bebop cameos existed until I saw four names in the credits, and figured it out from there. The hunt is on...