Looking for a manga where the main character goes from gentle, sweet and cute to evil or dark and mean and whatnot.

I would like it to be the MAIN character, not one of the supporting characters.
Example would be School Days Higurashi no Naku Koro ni

Mirai Nikki maybe, and not exactly what you want, but somehow similar: Bokusatsu Tenshi Dokuro-chan

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ççççççç[Ô .Ô] tsutopodus© will eat your manga and steal your cats!

Skip Beat fits your description to the last letter. =D

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I see what you are looking for there. Ummmmm.... shoot idk... maybe Shuffle?

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Nope, Skip Beat doesn't have Yangire...Mirai Nikki does. Yandere/Yangire is more about girls with psychological problems...to better describe, A Yandere is a quiet, shy type girl that goes Psycho (will kill rival/obstacle in her love life)...something like that...yangire is the same except the personality is more outgoing and cheerful like Yuno. Shuffle's Kaede is more Yandere than Yangire

In the first chapter of Skip Beat Kyouko goes pretty insane. She's trying to kill people with her dark aura alone. Soon after, she's stalking a guy and scratching at his window to the point he's going insane. Before that, she was the quiet girl who didn't talk much at all, however. She pretty much fits the description.

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Second Mirai Nikki and if you're going for anime (I never read the light novels,so I'm not sure about them) but Lina Inverse from Slayers tends to go haywire often too