Kotonoha Katsura is the yandere to Makoto Itou from School Days.
She went to Harami Junior High with Nanami Kanroji, but they were club members. Because she was popular with her male classmates, she was bullied by the girls. When she entered high school, she was placed in Class 4, the class next to Makoto's. 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. At some point, she started to develop feelings for him before Sekai introduced them to each other.
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 when she and Makoto are on a date. An extended version of Bloody End has Kotonoha attempting to kill Makoto for seemingly rejecting her, but she's unable to do so because she still loves him. In the anime, she suggests the Sekai get an abortion when the latter claims to be pregnant with Makoto's child, not wanting Sekai to bear any of Makoto's children. 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 her 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, content that they're together again.
In the manga, she tries to kill Sekai but ends up slashing Makoto accidentally.