Yui Kodai, hero name Rule, trains as a professional hero in U.A. High School's Class 1-B. She possesses shoulder-length black hair styled in a bob with side-swept bangs, fair skin, an athletic build, and consistently neutral cerulean eyes, standing 160 cm tall. Her hero costume consists of a red bodysuit with white collar trim and a V-shaped chest pattern, a short white skirt, white vambraces featuring yellow accents, and a distinctive red visor resembling Ultraman's design, complete with a fin-like metal piece at the back. Shoulder straps and a waist belt with pouches store small metal objects like nuts and bolts for her quirk; a grey oval gemstone adorns the collar. Exceptionally quiet and expressionless, Yui rarely speaks, communicating primarily through sounds like "Mm" or "Nn," particularly when happy or eating tomatoes. Classmates often describe her as a kuudere due to her reserved nature. Despite this, she demonstrates care for others, such as tending to Juzo Honenuki and Ibara Shiozaki after they were incapacitated by Mustard's poison gas. She maintains emotional stability and temper control but shows limited social skills and confidence. Her background reveals she was unaware of a fan club formed by male peers during middle school in Shimane Prefecture, even after graduation. Personal preferences include a fondness for tomatoes and matryoshka dolls, and she displays a notable disinterest in interactions with classmate Neito Monoma. Her quirk, "Size," allows her to alter the dimensions of any inanimate object she physically touches, enabling her to enlarge small items into weapons or shields or shrink large objects for portability. Activation requires pressing all five fingertips together after contact, with reversal achieved by repeating the gesture; the quirk does not affect living organisms. In combat, she employs long-range strategies, resizing thrown projectiles mid-trajectory or manipulating environmental elements. Her technical proficiency is rated highly, though she lacks direct offensive power. During U.A.'s Sports Festival, she placed 31st in the obstacle race and participated in the cavalry battle. At the forest training camp, she used her quirk to submerge underground and startle classmates during a courage test. In the Joint Training Battle against Class 1-A, she collaborated with Reiko Yanagi and Nirengeki Shoda, enlarging objects for Yanagi's telekinesis and Shoda's impact amplification, though Ochaco Uraraka ultimately captured her. In the Final War, Yui joined the operation to subdue Gigantomachia, working alongside Mina Ashido, Eijiro Kirishima, Minoru Mineta, Hitoshi Shinso, Juzo Honenuki, and Reiko Yanagi. She provided enlarged weapons for Mt. Lady, enhancing the giant hero's combat effectiveness against the villain. This collaboration continued during the confrontation with All For One until the villain overpowered the hero team. Eight years after the conflict, Yui graduated from U.A. and became a professional hero alongside her Class 1-B peers. Her prototype origins served as a basis for Mount Lady's early design, later redeveloped into a separate character. Her name combines the kanji for "small" (小), "big" (大), and "ordinary" (唯). She ranked 118th in the series' fourth popularity poll. Her hero costume's color scheme and crest pay homage to Ultraseven, reinforcing the size-changing theme. In the "U.A. White Day Festival" novel, she assisted with stage set resizing and portrayed Juliet in a class play.