Rei Ryūgazaki is a prominent character in the Free! - Iwatobi Swim Club narrative, serving as a core member of the Iwatobi High School swim team. He is distinguished by his tall, lean physique, dark blue hair, and the distinctive rectangular glasses he wears, which contribute to his initially serious and studious appearance. As a first-year student at Iwatobi High School, Rei joins the swim club later than his peers, and his background is notably different from the other members, as he was originally a track-and-field athlete. His decision to switch to swimming is not born from a lifelong passion but from a logical and aesthetic admiration for the sport, which he perceives as a beautiful and efficient form of movement.

Personality-wise, Rei is defined by his highly analytical and methodical nature. He approaches swimming with the same rigor he would apply to academic study, breaking down strokes and techniques into precise, scientific components. This intellectual demeanor often leads him to be pedantic or overly serious, and he can be blunt with his observations, which occasionally creates friction or humorous misunderstandings with his more instinct-driven teammates. Despite this, Rei is not cold or unfeeling; rather, his intensity comes from a genuine desire to understand and perfect his craft. He is deeply principled and holds himself to high standards, which makes him prone to frustration when his physical abilities do not match his theoretical understanding. Beneath his stoic exterior, he is also surprisingly sensitive, prone to dramatic gestures and emotional outbursts, especially when faced with the overwhelming passion and natural talent of his fellow swimmers.

Rei's primary motivation is the pursuit of excellence through logic and hard work. He is driven by a need to validate his choice to abandon track and field, a sport he was skilled in, for swimming, a sport where he initially struggles. His goal is not merely to win races but to swim in a way that aligns with his ideal of beauty and precision. This quest for a perfect, theoretically sound stroke is his personal challenge. As the series progresses, his motivation evolves as he becomes more integrated into the team. He begins to value the camaraderie and collective spirit of the Iwatobi Swim Club, finding meaning not just in individual achievement but in contributing to the team's relays and supporting his friends' dreams. His role in the story is that of the thoughtful newcomer who brings a fresh, intellectual perspective to a group driven by raw emotion and tradition. He often acts as a grounding force, asking the practical questions others overlook, yet he is also the one who most visibly struggles with the gap between theory and practice, making his journey a central arc about finding one's own path.

In terms of key relationships, Rei's dynamic with Haruka Nanase is particularly significant. He is initially baffled by Haruka's instinctual, almost mystical connection to water, which contrasts sharply with his own analytical approach. This relationship develops into a deep mutual respect, as Rei learns to appreciate the beauty of Haruka's natural swimming, and Haruka, in turn, is inspired by Rei's relentless dedication. His bond with Makoto Tachibana is one of gentle support; Makoto acts as a patient and understanding captain who helps Rei navigate the emotional and social aspects of the team. With Nagisa Hazuki, Rei shares a more playful and exasperating connection, as Nagisa's energetic and carefree attitude often clashes with Rei's seriousness, yet Nagisa is also the one who initially persuades Rei to join the club, and their friendship becomes a cornerstone of Rei's growth, teaching him to find joy in the sport beyond its technical aspects.

Throughout the narrative, Rei undergoes considerable development. He begins as a solitary figure, confident in his intellect but insecure in his physical abilities. His initial failures and the subsequent encouragement from his teammates force him to confront his pride and learn humility. He comes to understand that swimming is not purely a science but also an art and an expression of self. This realization is most evident in his adoption of the butterfly stroke, a discipline he initially chose because it was the most theoretically demanding. Over time, he masters it not just through formula but by injecting his own emotion and spirit into each movement, thereby achieving a personal synthesis of his logical mind and his newfound heartfelt passion. His development culminates in his acceptance of himself as a true swimmer, not because he matches his teammates' natural gifts, but because he has found his unique way to love and perform the sport.

Notable abilities of Rei include his exceptional analytical skills, which allow him to memorize and replicate swimming techniques with high fidelity after minimal observation. His background in track and field provides him with strong lower-body power, which he leverages for explosive starts and turns. His signature and most developed stroke is the butterfly, which he cultivates into a powerful and elegant technique that reflects his journey from rigid imitation to fluid expression. While not the fastest or most naturally gifted swimmer on the team, his precise form and strategic racing intelligence make him a formidable competitor and a reliable relay member, where his ability to maintain composure under pressure proves invaluable.