|Japanese name:||西園寺 世界|
She lives with her mother Yōko in the fictional area of Motehara-Sakashita and works part-time as a waitress at the family restaurant Radish.
Spoiler!In the anime, she attempts to set up Makoto and Kotonoha in a relationship, but ultimately ends up falling in love with Makoto. Makoto, bored with Kotonoha, decides to go after Sekai, who he impregnates after making love to her. Makoto, stressed and confused as to what to do about Sekai's pregnancy, leaves Sekai for Kotonaha. Sekai then stabs Makoto to death for his unfaithfulness to her, only for Kotonaha to kill her in return, which, in turn, aborted Sekai's fetus.
Her birthday and her surname are derived from Saionji Kinmochi. It's revealed in the Summer Days game that mother of Sekai is the daughter of Makoto's father (that makes her 'half-sister' of Makoto). So, Makoto is Sekai's uncle.
Voiced by: Shiho Kawaragi