Sekai Saionji attends Sakakino High School as a student in class 1-3. Raised in Motehara by her single mother, Youko Saionji, she never learned the identity of her father, disparaged by her mother as a "hopeless good-for-nothing" until discovering he was Shun Hazama. Her childhood involved frequent clashes with Niki Ookuma and a close bond with Setsuna Kiyoura. During middle school, she engaged in an inappropriate relationship with neighbor Noboru Matsukata, experiencing molestation she seemingly failed to recognize as abuse. Outwardly bubbly and energetic, Sekai presents herself as a supportive friend and enthusiastic matchmaker, often exhibiting boisterous or intrusive behavior. Beneath this lies significant emotional volatility fueled by deep-seated insecurities from her fatherless upbringing. She occasionally expresses jealousy towards peers with stable families and reveals a yearning for paternal connection during emotional lows. This insecurity manifests as possessiveness in romance, particularly concerning Makoto Itou, whom she initially researched for Setsuna before developing her own feelings. Her emotional fragility triggers prolonged depressive episodes, social withdrawal, and tearful isolation. Participating in a hot spring trip during the "Valentine Days" OVA, Sekai expresses disbelief at Kotonoha Katsura's family wealth, lamenting their economic disparity. She deceives Makoto and Taisuke Sawanaga by falsely claiming the onsen is mixed-gender. Later, at Radish restaurant, she denies planning to give Makoto "honmei chocolate" signifying romantic intent, insisting any gift would be obligatory "giri chocolate." Her mother observes Sekai's denials and offers chocolate-making guidance. On Valentine's Day, Sekai confronts Makoto alongside Otome Kato after seeing him with Kotonoha. This escalates into a global chase where Sekai pursues Makoto wielding a kitchen knife, eventually cornering him on the school rooftop with Kotonoha and Otome. The trio forcibly feeds him chocolates; Sekai interprets his request for deliberation time as a promise to reciprocate on White Day. Her characterization varies across media. The visual novel depicts greater selflessness; she distances herself from Makoto unless pursued and shows genuine remorse over affecting Kotonoha. She offers pragmatic relationship advice while lacking self-awareness about her own romantic entanglements. This version is less manipulative and better at maintaining non-Makoto relationships, though still vulnerable to severe depression upon rejection. The anime amplifies selfish and manipulative tendencies; she initiates a physical relationship with Makoto under the guise of "training" for Kotonoha while hiding her feelings. She sustains the affair despite recognizing its harm, rationalizing it after Makoto falsely claims to prefer her. Her mental state deteriorates following Setsuna's departure and Makoto's infidelities, culminating in psychosomatic pregnancy claims and her murder by Kotonoha on the school rooftop after Makoto is killed. The manga shows initial acceptance of Makoto's preference for Kotonoha, even smiling during their breakup. However, after Kotonoha stabs Makoto in an attack meant for Sekai, Sekai descends into violent yandere behavior, killing Kotonoha a week later. She then declares herself Makoto's sole source of future happiness while embracing his horrified reaction. In "Summer/Shiny Days," Sekai's role lessens due to illness, but suppressed jealousy surfaces when Setsuna reveals her relationship with Makoto. While initially supporting Setsuna's happiness, she later confesses superior love for Makoto after Setsuna leaves for Paris. "Cross Days" significantly reduces her presence, focusing mainly on her relationship with Makoto and depicting Yuuki Ashikaga's attempts to end their affair, including a near-assault prevented by Setsuna.