Sekai is the daughter of Youko Saionji and Shun Hazama. She is close to her mother, Youko, who is supportive but can be strict. Sekai's best friend is Setsuna Kiyoura, her half-sister. Other friends include Nanami Kanroji, a basketball player, and Hikari Kuroda, who is friendly and sociable. Sekai had a brief relationship with Makoto Itou, who was also involved with Kotonoha Katsura. Sekai is kind but can become obsessive if emotionally hurt.