Sekai Saionji is the one of the Yandere to Makoto Itou in School Days. The first is Kotonoha Katsura.
She's Makoto's classmate. One day, she finds out that Makoto had taken a picture of Kotonoha, which is a part of a charm to see if they were meant to be, she decides to help Makoto get together with Kotonoha. Despite her help, she also harbors feelings for Makoto, constantly fighting her desire to help him and to be with him.
Unlike her rival, another yandere Kotonoha Katsura, she's very possessive of Makoto. If her feelings are "returned" or toyed with, she starts to unravel and multiple things can happen as a result. In one of the Bad Ends of the visual novel and in the anime ending, she ends up killing Makoto for choosing Kotonoha over her. In another Bad End for the visual novel, she pushes Kotonoha in front of a train, killing her instantly.