2 case:

1) She is very supportive/always always by his side, but he doesnt see it until its too late or he likes someone else at the end.

2) The main girl will do anything for the guy that she likes, possibly using tactics like manipulation/murder. But he thinks shes crazy until its too late or he likes someone else at the end.
Bonus if:
She started out a normal girl but an event or something made her change

Thanks for odd request

2, but also 1, you meant something like School Days?. I can only think about Shuffle!, assuming that the manga follows the same route of the anime (don't have idea).

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What I like in Trivial Pursuit style. Pick your category:

Well I suppose Yamada would have done anything for Mayama in Honey & Clover but I don't think that's exactly the feel you're looking for. The only thing that comes to mind is as reid mentioned Shuffle!. Like reid I only watched the anime, and I think the whole thing is based off a game so I don't know how the manga goes.