Free Step Dodge

Yes. She is playable in both Ninja Gaiden Sigma and Ninja Gaiden Sigma 2. In Sigma 2 she's playable in one level and the mission modes. Okay, I guess. Not my style.
In Sigma 1 she is playable for several mandatory levels (3 or 4) added to the game that were not present in the Xbox versions (namely, Black). She is completely inept, annoying, her levels suck, is totally imbalanced

And is the biggest reason why I find Sigma barely playable, whereas NG: Black is my favorite action game of all time. Hayashi was an idiot for including those bits and I will never forgive TN for ruining one of the greatest games ever made via Rachel

My initial distaste and contempt for Rachel stems from that game (and the fact that when 5U first released she was hella-broken). Over time I've gotten over it and I'm more or less fine with her in DOA since the nerf (she's still a dumb character and absolutely braindead to use, but moderately fun and whatnot).
But I still don't forgive her for Sigma 1. Never will.

Well, which NG do you think Is best for me to get? (I may be able to get 2 of them). I was thinking of Razor Edge and Sigma 1 I think? But I feel your opinion is best lol

Do you have a 360 or just PS3?

I have 360 but it doesnt read discs so I can only get downloaded games on there. PS3 is fine.

Well, if you can get 360: Black is the best NG game hands down, and is my third favorite game of all time. Sigma (PS3) is theoretically quite similar to Black, but the changes are all for the worse and for those like me who hold Black so close to their heart, it hurts to play. But if you go in with a blank mind, it might be the second best after Black. Can't say that unbiased, though.

But in general NG1 is the msot technical and refined NG game in the gameplay department. It's slower, but still fast and while "difficult," it's almost always fair, challenging you to get better.

As far as the others go, it sorta depends what you're looking for. First off, NG3 is universally shit, so I'll only address RE.

NG2 (xbox) boasts some great landscapes and music, and introduced a lot of the modern mainstays of the NG titles. It's extremely bloody, extremely fast, extremely satisfying. It can jump into cheap category, but overall if you're good at the game, you'll get through and have a blast. And the weapons... dear God... so wonderful

Sigma 2 is the same game but they ramped down the gore and the difficulty, but added new characters, a few new areas, a co-op mode and a level select. There are pros and cons to each, honestly, and I'm not sure which overall I'd prefer.

RE brings back a lot of the craziness found in NG2 (xbox) with the speed and gore, but also adds the co-op mode, characters, level select etc.. But, it's also cheap as hell, highly unbalanced, has terrible enemy/level design, gimped weapons and abilities and a stupid upgrade system.