You know the whole Rachel is the only random member of the group (every one else was specially selected by ellimist) well it sort of fits because she explempifies humanity: reckless, violent and slightly insecure but still playful, intelligent and honourable.
You know, that makes fantastic sense.
Personally though, I see why a lot of people liked Rachel, but I found her a tad annoying, for one reason: predictability. "Let's do it" gets old after a while in my opinion, and stupidly saying "let's go for it" when everyone, herself included, knows whatever they are planning is hopeless or presents unnecessary risk.
I will agree that her dark side is very intriguing, but the general premise of her up front character is not very deep. Still, that doesn't mean that she's my least favorite character.