Sekai Saionji is a 16-year-old student in Class 1-3 at Sakakino Academy, born December 7. She resides in Motehara with her mother, Youko Saionji; her father, Shun Hazama, abandoned the family during her childhood. This paternal absence fuels her emotional instability and jealousy toward peers with stable families. Her family background involves multiple interconnected relationships through both parental lines. In childhood, she befriended Setsuna Kiyoura and Niki Ookuma in Motehara, but distanced herself from Niki due to frequent conflicts. Her past includes Noboru Matsukata, a middle-school neighbor whose initially innocent friendship escalated into molestation, though Sekai remained unaware these interactions crossed boundaries. She works part-time as a waitress at the Radish family restaurant for support. Physically, Sekai stands 155 cm tall with an average build. She has shoulder-length dark brown hair featuring a distinctive upward-curling "ahoge" and dark blue eyes. Her typical attire is her school uniform with white sneakers, or casual wear: a white midriff-baring top, pink jacket, and denim miniskirt. In colder weather, she wears a pink fur-trimmed coat bought with savings from her job. Severe emotional distress visibly thins her appearance, making her look sickly. Her personality varies across adaptations. In the original visual novel, she initially presents as cheerful, selfless, and supportive, helping Makoto Itou pursue Kotonoha Katsura despite her own feelings for him. She distances herself unless actively pursued and expresses guilt over their relationship. She perceptively advises others but struggles with personal emotional insight, especially regarding abandonment issues and depression triggered by Makoto dating Kotonoha or Setsuna moving abroad. The anime amplifies selfish and manipulative traits, showing ulterior motives in setting up Makoto with Kotonoha and continuing an affair with him despite the damage. Manga portrayals show her proactively pursuing Makoto from the start and reacting coldly to rejection. Core traits across all media include emotional volatility, jealousy, fear of loneliness, and depressive episodes causing days of isolation. Her relationships drive much development. She harbors deep-seated envy toward Kotonoha due to wealth, popularity, and physical appearance, despite initially befriending her to facilitate Makoto's romance. After beginning a secret sexual relationship with Makoto, she vacillates between urging him to confess to Kotonoha and clinging to him from fear of abandonment. Her friendship with Setsuna involves initial mutual support, but strains develop when Setsuna kisses Makoto and later moves to Paris. She maintains a close, confiding relationship with her mother Youko but resents her absent father. Romantic interactions with Taisuke Sawanaga occur in some storylines but end due to unresolved feelings for Makoto. In spin-offs like *Summer Days* and *Cross Days*, she occupies a less central role, sometimes sidelined by illness or narrative choices. The OVA *Valentine Days* portrays comedic rivalry with other girls over giving Makoto chocolates. Her character arc culminates differently by media. In the visual novel's branching paths, she may experience depression and isolation, reconcile with Makoto, enter a polyamorous relationship with Kotonoha, or suffer violent deaths: pushed onto train tracks or having her throat slit. The anime follows a linear progression where her affair with Makoto leads to pregnancy, his rejection, and her violent descent: she stabs Makoto to death in rage after he suggests abortion. Subsequently, Kotonoha lures her to a rooftop, reveals Makoto's decapitated head, and slashes Sekai's throat before disemboweling her to disprove the pregnancy claim. This ending emphasizes her transformation from a lively teenager to a victim of unresolved trauma and poor decisions.