Toshio Kotegawa owns and manages the diving goods shop Grand Blue, which also serves as his family residence. He possesses short white hair, brown eyes, and a tanned complexion, standing similar in height to Iori Kitahara with a build comparable to Kotobuki's, though less muscular. His appearance reflects a life spent in a coastal environment.

He maintains a consistently cheerful and laid-back personality, remaining unbothered by the disruptive behavior and frequent drinking parties involving nudity by the Peek a Boo diving club members who gather at his shop. This tolerance allows the club to use Grand Blue as their primary meeting space.

Family relationships define his background centrally. He is the father of Nanaka and Chisa Kotegawa and the uncle of Iori Kitahara. His wife works overseas, leaving him managing household responsibilities alone; her prolonged absence leaves their marital relationship undefined. As Iori’s guardian, he initially permitted Iori to live above the shop. However, after incidents involving Shinji Tokita, Ryūjirō Kotobuki, and Kouhei Imamura during an Okinawa trip provoked Chisa’s anger, Toshio enforced Iori’s eviction to maintain domestic harmony.

His interactions with his daughters appear rarely depicted, though tensions exist between Chisa and her mother due to past misunderstandings. Chisa received a gift from her mother affirming recognition of her diving skills, hinting at the family dynamics Toshio navigates. He seems supportive of Nanaka’s work as a diving instructor at his shop.

Within the narrative, Toshio primarily functions as an enabling figure for the diving club, providing their operational base through his shop. His tendency to rarely interfere permits their excessive behavior to continue. His background story lacks significant development, with his role remaining peripheral to the main plotlines centered on the club members across all media.