Kenichi Saitou is a middle-aged man who becomes a pivotal mentor to the protagonist, Koyuki Tanaka. In his past, Saitou was a competitive swimmer talented enough to be selected for the Japanese Olympic team, though an injury or an inability to achieve his ultimate goal of winning prevented him from reaching the pinnacle of the sport. At the start of the series, he is forty-four years old, unmarried, and runs a small business called Saitou Paper Industry. In his spare time, he frequently visits a local swimming pool where he takes it upon himself to offer rigorous, unsolicited lessons to those he deems unskilled in the water.

Saitou presents a dual personality that is starkly divided between his public and private life. When coaching swimming, he is famously loud, demanding, and eccentric, an intense taskmaster who drives his students hard. However, away from the pool, his demeanor changes completely. In relaxed settings, especially within his own home, he is much more open, calm, and even vulnerable. He often turns to Koyuki for advice on personal matters, particularly his unrequited feelings for a woman named Momo, revealing a loneliness and social awkwardness that contrasts sharply with his authoritative public persona.

A massive enthusiast of British rock music, Saitou's love for the genre is a central part of his identity. His favorite band is the fictional 1960s group The Rocket Boys, and he is also a devotee of legendary acts like The Beatles, Led Zeppelin, and Queen. This passion directly drives his role in the story, as he becomes Koyuki's first serious guitar teacher. When Koyuki accidentally breaks a borrowed guitar, Saitou happens upon him in his moment of despair and invites him home. There, he not only helps repair the instrument but also lends Koyuki one of his own prized guitars, a Gibson Les Paul, to practice on. This act of generosity marks the beginning of their deep, informal apprenticeship. Saitou provides Koyuki with not only lessons in guitar technique but also his first part-time job and the opportunity to perform at a local festival song contest.

Saitou’s relationship with Koyuki is the most significant in his life within the story. He acts as a surrogate guardian and a source of unwavering support, allowing Koyuki access to his extensive collection of guitars, including a cherished Gretsch White Falcon. In return, Koyuki becomes a confidant for Saitou’s personal struggles. Saitou also owns a pet cockatoo named Paige, whose name is a reference to Led Zeppelin guitarist Jimmy Page. One of his long-held, eccentric ambitions is to play his guitar well enough that Paige will sing along to the music.

In terms of abilities, Saitou is a competent and proficient guitarist, skilled enough to provide meaningful instruction to a beginner and to perform on stage. He is the leader of a group that enters a local music contest, demonstrating his practical experience. His background as an Olympic-level swimmer also makes him an expert coach in that discipline, and his technical knowledge of the sport is profound. Throughout the narrative, Saitou’s primary function is as an enabler and a catalyst, providing the tools, training, and emotional support that allow Koyuki to grow from a complete novice into a capable musician. His personal journey, while secondary, involves his awkward pursuit of romance and his subtle, persistent hope for his own second-hand dreams to be realized through his young student.