Unlike Kotonoha, Sekai has no reserves over attacking Makoto for causing her pain. But if driven to it she will lash out at Kotonoha instead.
While Kotonoha is usually in denial towards the pain Makoto causes her, Sekai lashes out at him instead.
In one bad route in the game, she was said to vanish years later, then confronted him. She reveales that she was pregnant with his child, but before allowing him to speak she violently stabs him in the stomach. But to her horror, he confesses to having loved her and was worried about her all of this time. After she runs away in a panic he bled to death while trying to make his way home to Kotonoha.
In another route she accidentally gets him killed after Kotonoha shoves her onto the train tracks and Makoto tried to help her. He manages to save Sekai, but gets hit by the oncoming train before he can get to safety.
The anime ends with Sekai killing Makoto for telling her off the evening before and causing her to snap when he demanded she rid of their unborn child. She requested they speak the following day at school and he agreed, so they met up at his place later the next day. He attempted to convince her to just have an abortion, leading to another arguement until he tries to calm her down. She left for the kitchen to grab drinks, then distracts him with a text before stabbing him several times with a kitchen knife, then runs home.
In the manga based on the first game and anime, it is implied she kills Makoto in the hospital, after informing him that she killed Kotonoha.
Her prioritized rival for Makoto and secondary victim. She is responsible for her death in Two of the games Endings:
The most common route of the two involves a broken Sekai shoving Kotonoha in front of a train, then cruelly watching as Makoto and the surrounding people start panicking.
In another ending she is implied to have killed her with Makoto. They meet up with their friends, who bring up not seeing Kotonoha for some time and express concern. They find it strange that neither Sekai or Makoto seem as concerned, then as they start to leave the area they bring up the recent report on a girls corpse being found in an alleyway and continue to note their strange attitudes.
In the first manga, Kotonoha attempted to kill her after being dumped by Makoto, hurt by the both of them for betraying her. But she accidentally attacks Makoto, sending him to the hospital and enticing Sekai's anger. Deeming her too dangerous, Sekai appears at Kotonoha's place and kills her, leaving the body for Kokoro, Kotonoha's little sister to find.