Blood type: B Also in year 1, class 3, she is Makoto's classmate. Sitting next to him in class, she sees the picture that Makoto has secretly taken of Kotonoha on his mobile phone. She then helps him get close to Kotonoha despite harboring strong feelings for him. Even though she is good-natured her emotions overwhelm her in many situations. She lives with her mother in an apartment. Known to be both a cheerful mood-maker and somewhat of a loudmouth. Her cute appearance and bright personality (as admired by Taisuke) attract a lot of secret fans, but with her frank and outspoken personality, all the boys who've confessed to her so far have been mercilessly rejected. In the last episode,
she kills Makoto after she finds out he has been cheating on her. Then, she realizes what she had done and feels remorse. Later on, she is invited by Kotonoha to go to the top of the school. There, they fight. Afterwards, she attempts to kill Kotonoha when she sees Makoto's head in a duffle bag, but Kotonoha kills her.Manga Spoiler
In the end of the Manga, Sekai stabbed Kotonoha but the situation isn't shown. Kokoro mentioned that Kotonoha is bleeding a lot while talking to Makoto on the phone, Sekai overheard the conversation and with a smirk she confessed to her crimes to Makoto that after Kotonoha's attempt to murder Sekai, she isn't worthy enough to be Makoto's Girlfriend.