At the age of 16 years old, she already had mastered method acting, the form of acting that involves recalling events from one's own past to evoke the emotions required for a role. Her skill made Sumiji Kuroyama invite her to his studio and guide her to be an actress who will leave an imprint on history of acting.
Kei is a young girl with mid-long black hair that reaches her back with a curvy long ahoge. She has fair skin and her height is 168 cm also, her eyes are lash black eyes. Her body type is averaged for her age. In the beginning, she wears a green and white sailor high school uniform with black socks and brown shoes.
Kei has a somewhat unusual and strange personality for a girl her age. Before her debut as an actress, her siblings Rei and Rui described her nature as "creepy" and "scary" and encouraged her to pursue acting as an excuse for her weird behavior.
- Yonagi's favorite foods are fish, natto, japanese food like Hijiki and western food like hamburger or curry.
- Her special skills are sprint, long jump and general exercise for individual competition.She isn't good at ball games.
- Her favorites movies are Roman Holiday, Casablanca, and Gone with the Wind.