Kei Ninomiya is Natsuo's friend from the volleyball team, and possibly Natsuo's only friend. Kei possesses a mostly polite and delicate demeanor when addressing others. She seems to really care about Natsuo, stating she was worried about her before she started doing MMA due to her lack of motivation, showing joy upon seeing her excited about something, immediately calling and yelling in excitement at her after Natsuo texted her for the first time, and coming to talk to her during lunch every day even though she is from a different class. However, she also shows brutal honesty when talking to her, saying she has “her fair share of problems”, “a terrible personality” and stating she hates the side of her that acts superior to others.
Kei seems to possess average height, topping around Natsuo’s shoulder. She has medium length hair that goes past her shoulders, and is usually shown with a pair of pigtails that she ties with white ribbons by the sides of her face.
When in school, she is seen wearing her school uniform, composed of a tucked white shirt, a striped tie, a dark skirt, and short, white socks. She commonly wears a plain vest on top of it. When dressing casually, she’s shown in comfortable dresses with simple adornments.
- She started calling Natsuo ‘Na-chan’ under Natsuo’s suggestion.
- She admires Steven Seagal's stunts.
- Natsuo often shares her lunch with her.