Sayaka Kouno from Princess Princess...
She is in love with he cousin Toru. To the point that she want to marry him.
Pushed a girl that was with him down the stairs and Rigged a letter so it will cut the fingers when the receiver tries to open it...
But she gets better by the end of the series
And another...But I'm not sure if he should be counted as one...Because it just happened recently.
Aichi Sendou from Cardfight Vanguard
For 39 episodes he was pretty normal, though he had a strange obsession/relationship with Kai. He looks up to Kai like an older brother and wants him to recognize him as a worthy fighter. But when Aichi starts using his ability during fights, he starts showing "Yan" traits. (Creepy faces and smug remarks)
Then afterward, acts like it was a normal fight. After one battle, he goes up to Kai and smiles to him and reported that he won...But afterward Kai tells him that he's weaker than before, which makes Aichi upset. Aichi challenges him to a fight just to prove to him that he is stronger than before, but Kai declines and leaves him, which makes him more upset because it was Kai that brought him into the game.
Repeat forever. Promise forever
That you'll keep me in your heart.
well actually she started to attack Mai unconsciously (through some ghost-like Hime-User) while she (Shiho) was sleeping in hospital because of her enviousness about Mai beeing more loved by Yuuichi Tate. Then in ep.24, she started a battle against Mai knowing of her Hime-powers (quote:"It's all your fault... so please.... dissappear" ... "because oni-chan only pays attention to you.").
another possible yandere may be Maki Tokitou from Karin, ep 20: she was sitting on a bench, ripping the filaments from her self-made scarf for Winner Sinclair and started to tighten them, so to be used to strangle someone, in this case Karin because Winner only pays attention to her. But this didn't happen because of Anju, Karin's little sister and blood-sucker "of grudge", who bit her, so she went back to her "normal" self.
This is a main part of the plot so I don't know if it counts as a spoiler, but, just in case, Yomi becomes friends with a girl named Mato, and then eventually becomes jealous when Mato makes a new friend, and turns into this demon-like girl named Dead Master
Sayaka is an interesting sugguestion, but I think it was her dispair at finding out the truth about her "contract" Her becoming a witch and what not I don't think are a result of her feelings for the boy. I agree with the club mods. I kind of wonder about Tsukihe Araragi, she saw a girl in the bath with her brother and came back with a large knife lol. She just maybe inherently violent though
Celes is definitely not a yandere/yangire, her behaviour has nothing to do with jealousy or irritation towards any of the characters in the story, she's just a two-faced liar. I'm not sure about Touko, she doesn't kill anybody at the school. There's only a knowledge that she did kill before in the outside world because of her fetish for pretty boys.