Yui Kodai, hero name Rule, hails from Shimane Prefecture and enrolls as student number 6 in U.A. High School's Class 1-B. An unnoticed fan club formed by boys during her middle school years persisted after her graduation, unknown to her. She graduates alongside her peers following the Final War, becoming a professional hero. She possesses a shoulder-length black bob cut swept to her right, with shorter strands above her cerulean eyes, fair skin, and an athletic build, typically maintaining a neutral, expressionless face. Her hero costume consists of a red bodysuit with white collar trim and a V-shaped chest pattern, a short white skirt with red accents, and white vambraces featuring yellow details. A distinctive red cap echoes Ultraman’s design, complete with a fin-like metal attachment. A grey or yellow oval gemstone adorns her collar, and shoulder pouches carry nuts, bolts, and other small metal objects for her Quirk. Yui exhibits a reserved, quiet personality, rarely speaking or showing emotions, leading some classmates to label her a "kūdere." She communicates minimally, using sounds like "Mm" or "Nn!" for agreement or happiness, and an emphatic "MM!" while eating her favorite tomatoes. Despite her stoicism, she demonstrates care for others, such as tending to Juzo Honenuki and Ibara Shiozaki after Mustard’s gas incapacitated them at the Forest Training Camp. She avoids attention, maintains distance from most peers, and shows particular coldness toward Neito Monoma. Her calmness and composure under pressure shine through. Her Quirk, Size, enables her to alter the dimensions of inanimate objects upon physical contact. Activation requires touching her fingertips together, and the effect reverses similarly. She commonly transforms pocket-sized metal pieces into enlarged weapons or barriers or shrinks large objects for easy carrying. While lacking direct combat versatility, the ability excels in tactical support; she collaborates effectively with Reiko Yanagi’s telekinesis and Nirengeki Shoda’s force amplification during the Joint Training Battle. In the Final War, she aids Mt. Lady by supplying enlarged weapons against Gigantomachia, though their group is later overpowered by All For One. Her Ultra Analysis Book stats rate Technique at A+ and Coolness at S, reflecting her Quirk mastery and unflappable demeanor. Key involvements include the U.A. Sports Festival, where she placed 31st in the obstacle race and joined a cavalry battle team. At the Forest Training Camp, she used her Quirk to sink into the ground during a courage test, attempting to startle classmates. During the Joint Training Battle’s fifth round, her coordinated attacks with Class 1-B teammates were countered by Ochaco Uraraka, resulting in their loss. She participates in missions against Gigantomachia and villains during the Paranormal Liberation War and Final War arcs, emphasizing her role in large-scale operations. Her birthday is December 19, blood type B, and height 160 cm. She enjoys tomatoes and matryoshka dolls. The Ultraman-inspired hero costume incorporates homages like Ultraseven’s Eye Slugger crest and a Color Timer-like gemstone. Her conceptual origins link to early prototypes of Ochaco Uraraka.