I hate Namine! >.>; Well...not really, but I do like Kairi a hell of a lot more. Namine was not "forced" to mess with Sora's memories, but she didn't "want" to do it either. It's a little of both; at least it is in my opinion. I think Marluxia just manipulated her by saying stuff like "Are you lonely? You don't have to be...I'll help you find some friends if you help me," or something to that effect, but I doubt he threatened her life or anything. So that way, she didn't exactly "want" to do it, but at the same time she did so she would be his friend and wouldn't be lonely.
How does this explain why I don't like her as much as Kairi? She's so weak and easily manipulated. She comes across ( at least in my perspective ) as a character that, if pushed a little, would do harm to others to help herself; in this case, to stop her loniless. Don't get me wrong though...at heart she's a good person who's just meak and shy. Since she'll probably be with Sora sometime in the game, I bet by the end of KHII she'll have a stronger heart from being with him.