Koki Matsuda is a high school student and singles badminton player at Yokohama Minato High School, sharing a class with Ryo Mizushima. Having lived in China from age eight until the spring of his junior high year, he returned to Japan as a returnee. This international background left him unrecognized in Japanese badminton circles upon his return, despite wielding significant talent.

Physically, Koki possesses black hair, black eyes, and a well-developed, handsome facial structure frequently associated with his popularity. He competes in a standard blue badminton outfit. His personality is defined by a consistently cool, unemotional demeanor; he approaches situations rationally and acts based on logical assessments rather than emotion. This temperament aligns with his Cancer zodiac sign and blood type B.

Within the badminton team, which includes players like the Higashiyama Twins, Shohei Sakaki, and Kento Yusa, Koki's presence introduces a composed, analytical dynamic to interactions and training. Outside the court, he maintains a fan club among his classmates.

Additional details include his height of 173 cm, a birthday on July 21, and a preference for homemade hamburgers. The etymology of his name reflects his character: "Koki" combines "航" (navigate/sail) and "輝" (brightness/radiance), while "Matsuda" means "pine tree" and "field/rice paddy".