Izumi Yunomoto is a Hakuoh Academy student and member of its Space Yacht Club. She possesses golden eyes and brown hair combed away from the right side of her forehead, tied in a low back ponytail, and wears large oval glasses. For school, she wears the standard Hakuoh Academy uniform, sometimes substituting a jumper for the jacket. During club activities, she wears a green hoodie featuring the club's emblem—a yellow crescent moon with a star—on the upper left chest.

Her background before joining the yacht club is largely undocumented, though she joined sometime prior to the series' events. She maintains close friendships with fellow club members Berinda Percy and Asta Alhanko, frequently collaborating with them on club operations. Her personality is friendly and good-natured, matching the approachable nature of the yacht club members.

Izumi consistently handles sail system operations on the club's training vessel, the Odette II. In the Recruitment Arc, she operated sail consoles during a practice cruise, confirmed pressure readings during critical maneuvers, voted on flight paths, and maintained her station during electronic attacks and enemy confrontations. For the Golden Ghost Ship Arc, she assisted in planning practice cruises to cover covert operations.

During the Hakuoh Pirates Arc, she supervised first-year students in simulators, joined the crew in temporarily commandeering the Bentenmaru pirate ship, and wore a nun costume during the raid on the luxury liner Princess Apricot. She later participated in rescuing former club president Jenny Dolittle, boarded the Glorious Coolph ship as part of a protective detail, and celebrated successes with the group.

In anime-exclusive arcs, Izumi observed the club's Nebula Cup qualifying race but was assigned physical drills instead of competing. During the Pirate Hunter Arc, she supervised dinghy simulations while preparations were made for a farewell cruise.

In the film *Abyss of Hyperspace*, Izumi assisted with recruitment preparations during spring break, met Kanata Mugen, helped solve a puzzle left by his father, and participated in a decoy operation to mislead the Yggdrasil Group. When compromised by surveillance, she joined electronic warfare aboard the Odette II during a subspace battle, supporting the Bentenmaru.

Novel-exclusive arcs depict Izumi involved in advanced club activities: responding to a transmission from the lost ship Kokuchou, thwarting a theft attempt on the Odette II, and navigating frontier space battles alongside privateer forces. She later received an invitation to the Pirate Guild.