At the age of 16 years old, Kei had already mastered method acting, a form of acting that involves recalling events from one's own past to evoke the emotions required for a role. Her skill inspired Sumiji Kuroyama to invite her to his studio, Studio Daikokuten, and guide her to become an actress who would leave an imprint on the history of acting.
Kei is a young girl with a slim figure and fair skin. She has chest-length black hair that arches outwards near the ends, framed with a single, long ahoge that curves around her face. Her eyes have prominent lashes and are a deep black color, often with red or purple accents.
In her first appearances, Kei wears a long-sleeved teal-and-white sailor uniform with black socks and brown shoes. Upon returning back to school after performing in Night of Galactic Railroad, she switches to her Winter uniform—a black version of her seifuku with red accents.
Kei's casual attire consists of various graphic t-shirts and shorts.
Kei has a somewhat unusual and strange personality for a girl her age. Before her debut as an actress, her siblings, Rei Yonagi and Rui Yonagi, described her nature as "creepy" and "scary," encouraging her to pursue acting as an excuse for her weird behavior.
- Kei's favorite foods are fish, Japanese foods like hijiki and natto. When cooking for her siblings, she also prepares western foods like hamburger or curry.
- Her special skills including sprinting, long jumping, and many other sports that can be done individually. However, she isn't good at ball sports.
- She enjoys romance films, her favorites being Roman Holiday, Casablanca and Gone with the Wind.