Kōji, also called Kōri-chan, is a young ice devil with long ice-blue hair, black-blue eyes, and a button nose but no visible mouth. She typically wears a very light blue kimono. Her biological makeup produces strawberry-flavored blood and lemon-flavored urine. Nonverbal, she communicates without speech, and her facial structure occasionally resembles a mole's.

As the younger sister of Yusa, a yuki-onna who cares for her and sells shaved ice, Kōji receives protective support; Yusa confronts others over perceived mistreatment. Kōji dislikes being demeaned as a "low-class" demon and retaliates against belittlers like Jashin. She maintains positive interactions with Yurine Hanazono, who aids her during confrontations.

Kōji endures recurring distress from multiple kidnappings by Mei Tachibana, a police officer who collects cute beings, leading to fearful reactions around Mei. Despite this, she displays moments of happiness, especially with Yurine. Her chronological age approximates one human year.