Any I dont know how to unlock a single one.
There is a CG of the Rat King and Tao which is a chance encounter when you enter his lair. I can;t remember, but I believe that other CGs with the rat king are available through servicing him. There is also one with Korg and Hayden which is unlocked by bringing Korg to the hotsprings. These are the only other two I can think of... There might be one if you lose against Scar and get locked up as a fucktoy for the other bandits but that might just be a vivid text scene.