Sekai Saioniji

Stub Spoiler

Sekai Saioniji is a girl from the infamous anime series known as School days, she is a student in year one, class three who is an antithesis foil of Katonoha. A playful and outgoing classmate seated directly next to Makoto, 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.

Anime
In the anime she is trying to set Makoto and Katonoha in a relation but ultimately fails when Sekai fell in love with Makoto, She often prevents Makoto to come back to Katonoha 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 Katonoha and Makoto are together she got devastated and stabbed Makoto to death on later to get killed by Katonoha fatally in the neck and stomach.

Manga
In the manga, Katonoha 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 Katonoha to prevent her from harming Makoto.