Riku Matsuzaki lives at Yokohama's Coquelicot Manor boarding house as the youngest child in the Matsuzaki family. He is the son of Ryoko Matsuzaki and the late sailor Yūichirō Sawamura, who perished in the Korean War. His older sisters are Umi Matsuzaki, the narrative's central figure, and Sora Matsuzaki. After his father's death, Riku remains the sole male in the immediate household.

Riku possesses brown hair and dark brown or black eyes, standing slightly taller than his sisters. He typically wears standard school attire.

In family interactions, Riku embodies younger-sibling tendencies. He demonstrates impatience, pressing others to begin meals without waiting for absent relatives. Leisure activities like television viewing take precedence over chores; he refuses errands such as buying dinner pork due to TV preoccupation. He exhibits a hearty appetite, consuming substantial portions at mealtimes.

His narrative involvement remains peripheral. Riku does not engage in core plotlines concerning the Latin Quarter clubhouse's preservation or familial revelations impacting Umi and Shun Kazama. His scenes occur solely in domestic contexts, featuring brief mealtime conversations or recreational moments with his grandmother and sisters.

His given name "Riku" (陸) means "land" in Japanese, thematically contrasting with Umi's "sea." The surname Matsuzaki fuses "matsu" (松, pine/fir tree) and "saki/zaki" (崎, cape/peninsula).