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 in a fit of rage after she finds out he has been cheating on her without any sort of guilt or remorse. Afterward, she feels regret toward what she had done.

Later on, she is invited by Kotonoha to go to the top of the school where Sekai tells Kotonoha that she is pregnant and it is revealed that Kotonoha had decapitated Makoto's corpse and was carrying his head in a duffel bag. Horrified, Sekai comes to the realization that Kotonoha was driven insane. Kotonoha asks Sekai if she can check if the latter is pregnant wherein she brandishes a hacksaw. Sekai attempts to defend herself with the same knife she used to kill Makoto, but is disarmed and fatally slashed across the neck. Kotonoha then proceeds to slice open her abdomen to check if she was indeed pregnant (apparently, she was not).

This is the last time Sekai appears in the anime.
Manga spoiler:
At 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.