Ai Ninomiya is a seventeen-year-old student at Yumegaoka High School and a key member of the school’s Diving Club, where she serves as president. She is one year ahead of the main protagonists, Hikari Kohinata and Futaba Ooki, and acts as their mentor and senpai, guiding them in the techniques and joys of scuba diving. Ai is the older twin sister of Makoto Ninomiya; despite being fraternal twins and in the same grade, she is the dominant sibling, often kicking him, ordering him around, and using physical humor to vent her frustrations, though she genuinely cares for him beneath her tough exterior. She has pale skin, vivid green eyes, and thick pinkish-red hair that she usually styles into two pigtails, using three bobby pins to keep her bangs in place. Her personality blends spirited, upbeat, and hot-blooded energy with a surprisingly down‑to‑earth and mature side, especially when caring for younger club members. A notable quirk is her low blood pressure, which makes mornings difficult and takes her about three hours to fully wake up. She can also be indecisive at times. Throughout the series, Ai’s role expands from comic relief to showing empathy, romantic sensitivity, and quiet dedication to her brother and friends, all while leading the Diving Club with experience and skill.