I think that Cassie was the nice girl who slowly transitioned, obviously not into a crazy Mean Rachel-type, but did become a bit more hardened and ruthless. Well, everyone did, in a sense. Like in #32 when Nice Rachel realizes that everyone has a Mean and Nice part of them, and in her the Mean part was just growing and growing. So it is more interesting, imho, that Rachel was a bit tougher than the others from the start.
Okay, I'm totally incoherent today, but hopefully that made some sense.