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Sekai Saioniji is the main antagonist in School Days. She is a student in year one, class three who is an antithesis foil of Kotonoha Katsura. A playful and outgoing classmate seated directly next to Makoto Itou, she takes a curious interest in him during their first acquaintance, and in every medium of the series, helps to establish his bond with Kotonoha.
In the anime she is trying to set Makoto and Kotonoha in a relation but ultimately fails when Sekai fell in love with Makoto, She often prevents Makoto to come back to Kotonoha in fear of losing him, She often caught Makoto cheating on her with other girls, including Setsuna (Sekai's best friend), when Sekai finds out Kotonoha and Makoto are together she became devastated and stabbed Makoto to death. In vengeance Kotonoha fatally stabs her in the neck and stomach.
Sekai is initially very supportive of Makoto's relationship with Kotonoha, but she quickly realizes that she wants him too. She declares her love for Makoto on a beach, and from that point on tries to get him to cheat on Kotonoha with her. The initially reluctant Makoto gives in, and Kotonoha breaks down when she sees the two dancing on the roof.
Kotonoha very nearly kills Sekai for luring Makoto away from her but accidentally slashes Makoto himself, who gets admitted to a hospital, she then got insane in shock of being killed and killed Kotonoha to prevent her from harming Makoto. She's last seen lying on top of a horrified Makoto, saying that they'll be together forever.
- Unbeknownst to her and Makoto the two are related with Makoto being Sekai's double half uncle.