Yui's appearance is something of a visual cross between Yoko Ritona of Tengen Toppa Gurren-Lagann and Signum of Mahou Shoujo Lyrical Nanoha. She's physically normal for an athletic girl her age, standing 5’10” tall and weighing in at 130 pounds. She has an athletic frame and a fair but not intrusive bust size. Yui's hair is of a deep ruby-red hue, flowing down to her waist in the back, while her eyes are of a sky-blue color. When not in her orange coveralls or pilot suit, Yui wears a red-and-white set of clothing - as befitting her tomboyish nature, she wears a male set of clothing rather than female, consisting of a white shirt under a red vest and matching pants. Her other bit of attire is a fire opal brooch around her neck, designed with a silver depiction of a phoenix curled around the opal like a giant egg.
Somewhat oddly for a mechanic, Yui is extremely fiesty and hotheaded, quick to anger and with a temper like a firecracker. Underneath this lies a current of sincere depth and kindness, which begins to rise to the fore over time as she begins a romantic relationship with Alex - if someone is in pain, she won't hesitate to comfort them, and will shed her temper if someone is crying. She's an exceptionally skilled mechanic, able to build entire ATs nearly overnight.
- Pheora: After being called a "chick", Yui builds Pheora overnight out of raw fury. It serves as her personal unit after its deployment in the next day's battle proves its effectiveness, and she uses it for the remainder of the war.