Koto Ōtsuki is a supporting character and childhood friend to Eiji Busujima, acting in an older-sister role despite being only two years his senior and attending the same high school. She maintains a playful, mischievous demeanor, frequently teasing both Eiji and Nanako Yukishiro with witty, often raunchy innuendos.

Physically, she possesses a notably developed figure and strong physique, characterized by pale skin, straight chest-length pink-violet hair tied in a ponytail with a red ribbon, square bangs framing her forehead, and wide blue eyes. Her standard attire is the school uniform.

As president of the "Modern US army combative" club, she represents a significant disconnect between her appearance and her club affiliation, ranking first among peers in this regard. While outwardly supportive of Nanako and Eiji’s developing relationship, she privately harbors romantic feelings for Eiji. She provides romantic advice to Nanako when solicited.

Her surname "Ōtsuki" combines the characters for "big" (大) and "moon" (月), while her given name "Koto" means "Japanese harp" (琴).