Kotomi Tsuda, younger sister of Takatoshi Tsuda, enrolls at Ousai Academy due to its proximity to their home. She faces academic struggles, consistently earning C grades in her classes. Her personality blends high energy and an outgoing nature with genuine consideration and care for others. Kotomi exhibits notable curiosity about adult matters, frequently making suggestive comments to embarrass her older brother, including implications of inappropriate physical involvement between them. This aligns her with tendencies like Shino Amakusa's, demonstrated when she brought adult manga to Takatoshi during his illness. She forms a significant friendship with Kaoru Toki, her first high school friend, actively encouraging the seemingly intimidating but clumsy and non-rebellious Kaoru to "lighten up" and follow school rules. Kotomi interacts easily with peers, connecting effortlessly in social settings.