(Obnoxious/Loud and expressive)
Sekai is in the same class as Makoto Ito and her best friend, Setsuna Kiyora. When first starting high school, she fell in love with Makoto who would later be assigned the seat next to her in class. She is notorious for being annoying, nosy, and very loud at times. Though Sekai was a cheerful girl who appreared to have cared deeply for her friends, she actually turned out to be an extremely selfish, manipulative, uncaring, brash, impulsive, unremorseful, and demanding girl, shown as when she kissed Makoto Ito when he was about to hop on the train, despite arranging a date for him and Kotonoha.