Yumiko Koizumi is a talented junior golfer, initially introduced as a champion in girls' junior golf tournaments. She earns recognition for her powerful iron shots and exceptional backspin technique, gaining the nickname "Iron Master." Her average driver distance is 195 yards, with an average of 2.1 putts and a best score of 1 under par. She enters the Camelot Academy Japan branch as a fifth-grade student, quickly gaining attention for her skills and securing promotion to the competitive A Class. During her early academy days, she emerges as a formidable rival to main characters Gawain Nanaumi and Lancelot Norman, participating in official matches and contributing to intense competitions. She develops a crush on Lancelot Norman but loses interest following a thoughtless comment from him, contrasting with other characters exhibiting more persistent romantic devotion. Over time, her passion for golf diminishes, leading to a decline in performance and demotion to B Class, where she becomes an average student. Her initially cheerful and lively personality shifts toward carelessness; she adopts a dismissive attitude, belittling classmates such as Quester Phoenix and Tristan Liones and downplaying their efforts as "childish" or "misunderstandings." This change culminates in Lancelot Norman openly criticizing her sloppy approach, serving as a wake-up call that prompts her to reevaluate her behavior. Following this confrontation, she undergoes significant development, rebuilding her dedication to golf through renewed hard work. By the story's conclusion, she reemerges as a professional golfer, competing at a high level and frequently facing Percival Lawrence in tournaments, with their skills described as evenly matched. Her appearance evolves alongside her character: she initially possesses long black hair (described as brown in some sources), stands 154 cm tall, and wears a school uniform, later adopting a very short hairstyle during her professional career. Her personal background includes an older brother who appears in the story, though she shows little interest in him despite his presence.