Komari Koshigaya is a second-year middle school student living in the rural village of Asahigaoka. She is the younger sister of Suguru Koshigaya and the older sister of Natsumi Koshigaya, making her the eldest girl in her family and at the local branch school. Her most prominent struggle is her intense desire to be perceived as a mature adult, a goal that is constantly challenged by her notably short stature. She is acutely self-conscious about her height, which often causes others to mistake her for an elementary school student. This insecurity is matched by a personality that is easily flustered and highly impressionable; she is quick to become frightened by supernatural stories, horror themes, and superstitions, a weakness her younger sister Natsumi frequently exploits for amusement. Despite being easily scared, Komari is fundamentally responsible and kind-hearted, and she tries her best to look after her younger friends even when she feels out of place.

Her relationships form a core part of her character arc. She has a close but turbulent bond with Natsumi, who teases her relentlessly. With Hotaru Ichijou, she experiences a complex mix of admiration and jealousy, as Hotaru is younger but taller and often appears more composed. Hotaru deeply adores Komari, viewing her as exceedingly cute, a dynamic that Komari finds both flattering and frustrating. With Renge Miyauchi, she shares a gentle friendship, though she strongly dislikes being called by the nickname Koma-chan, which emphasizes her small size. Komari's development throughout the series revolves around her journey toward self-acceptance. She initially struggles with domestic tasks, notably cooking, and pretends to have knowledge about teen trends like fashion, music, and romance that she does not actually possess. Over time, she learns that true maturity involves recognizing one's limitations and asking for help when needed. She improves her cooking skills and slowly becomes more comfortable in her own skin, balancing her aspirations for adulthood with the joy of her present childhood experiences.