Kotonoha Katsura is the yandere to Makoto Itou from School Days.
She was a classmate of Makoto's, but they were in separate classes. He had seen her on the train a couple of times and he decided to test out a theory by taking a picture of her with his phone.
In the original game, it's possible to avoid turning Kotonoha into a yandere by being nice to her and not toying with her affections. However, certain actions that lead to her breaking will trigger a bad end. Bloody End, for example, shows Kotonoha breaking into Makoto's apartment and hearing him have sex with Sekai multiple nights. She refuses to believe that she is no longer his girlfriend even when another classmate asks her about her relationship with him. One night, she spots a handsaw outside his apartment and decides to kill Sekai the next day by slicing her neck with it. In the anime ending, she comes home to find Makoto's body after Sekai violently stabbed him to death. She cuts off his head, contacts Sekai and uses Makoto's phone to lure Sekai to the school rooftop. She makes her look at Makoto's severed head before slicing her neck and cutting her open to see if she was pregnant with Makoto's child. The series ends with her setting out to sea and cuddling Makoto's head.