Ayumi Oumi is the president of the astronomy club at the prestigious Nogishiro High School. Her background is initially presented with an air of mystery; she has a composed and formal demeanor that makes her difficult to read. She is first encountered when her school hosts an Autumn Stargazing Conference, an event that brings together astronomy enthusiasts from multiple schools. Ayumi is a capable and organized leader, as she is responsible for explaining the scientific process of the conference, which involves splitting into groups to count visible stars in a patch of sky after the moon sets to measure sky brightness. Her personality is a blend of quiet reserve and sharp intelligence. She has a distinctive appearance, noted for her dark, somewhat mysterious eyes and a style of dress that is more conservative than typical for a high school student. This composed exterior hides a pragmatic and perhaps slightly lonely reality; she later confides to Saku Ooyagi that despite the large turnout at the conference, her own club actually consists of only nine genuine members, with the rest being borrowed from other clubs to give an impression of strength.

Ayumi's role in the story is to serve as a catalyst for other characters and to provide a glimpse into a more structured and academic side of amateur astronomy. She interacts directly with the protagonist, Saku, asking for his assistance in patrolling the school grounds during the event. This interaction reveals her ability to identify and utilize capable individuals. Her most significant relationship is with Takeyasu Roma, a primary character from the main cast. Ayumi has a personal history with Roma, as she reveals that they were once members of the same astronomy club at the Goshima Science Museum. She confronts him directly about this shared past, demonstrating that she is not intimidated by his eccentric personality and that she holds a personal stake in their connection. Her motivations are tied to a genuine passion for astronomy and a desire to maintain a functioning, credible club despite its small size. She shows concern for others' futures, as she is later seen noticing Roma stressing over university entrance exams and takes the initiative to help him, such as taking him to buy study books.

Over the course of her appearances, Ayumi demonstrates a notable ability to manage logistics and lead a scientific observation event, showcasing her knowledge of practical astronomy techniques like star counting for light pollution measurement. Her development is subtle but evident; she transitions from a seemingly mysterious and formal rival club president to a more relatable figure with personal ties to the main group. She is in the same graduating class as Roma, and following his graduation, she is depicted as having entered university alongside him, indicating a continued bond beyond high school. Notably, she does not possess any fantastical or combat-oriented abilities, but rather excels in organizational leadership, scientific methodology, and social strategy within the context of a school club.