Yui is a young girl with shoulder-length, bob-shaped hair, sideswept to her right with some shorter strands hanging above her eyes. She has fair skin, an athletic build, cerulean eyes, and she's usually seen wearing a neutral expression on her face.
Her hero costume consists of a simple red bodysuit with a white collar trim, a matching ”V” pattern running over her chest and around her upper arms, which also includes a short white skirt with a red design at the front, dipping so it follows the one in her chest. She wears plain vambraces, as well as a hat which resembles the front of a baseball cap, colored red with white U-shaped lines around the sides, with a metal piece resembling a fin attached at the back of the strap around her head. She has a belt at her waist and straps around her shoulders, what appears to be an oval-shaped gemstone set between them on her chest, each adorned with a small pouch, containing the various nuts, bolts and all of the other small metal pieces she likes to use with her Quirk.
Yui is a quiet girl that rarely expresses any emotions; some of her classmates even consider her a kūdere. Despite her silent demeanor, Yui has shown to care for her classmates, most prominently during the Forest Training Camp Arc, when she looks after her Juzo Honenuki and Ibara Shiozaki after they get knocked unconscious by Mustard's poisonous gas.
Although she rarely talks and has no expression, Yui isn't moody. When she's happy, she says 'Mm!' When she eats her favorite tomatoes, she says 'MM!'