Help me find the name for this anime?

Somewhere in the mid two thousands I remember watching an anime on cartoon network, I can't really remember if it was a movie or series. Anyway the main character was a robot boy. At one point he meets a girl who's trying to help him. . . hide something from a villain? And she talks a lot about her past with her maybe mother or grandmother. She had this special place where she spent time with her grandmother in the past, I think.

So they go there but then it turns out that she was a robot, even though she had no idea. And that the villain had planted those memories in her mind so they'd go there. Then the girl, now in robot form, tries to fight off the villain, or some such thing, and ends up. . . decapitated?

I know for certain that their robot forms were fairly humanoid (as in they had like one piece thing shaped like their hair and the women had tiny waists and some sort of a bust), but in bright colours. There was also this other. . .hot pink? robot girl, I have no idea whose side she was on but she spent some time up a tree. . . maybe taunting the main character? I'm pretty sure she had pigtails. And I think the main character himself had an orang-ish robot body, but I'm not certain.

Does this ring a bell for anyone?