Rei Ryugazaki entered the Iwatobi High School Swim Club with a strict non-swimming condition, dedicating himself solely to theoretical elements like club formation and stroke mechanics. As a competitive swimming novice, he pursued the demanding butterfly stroke despite its difficulty, starkly contrasting Haruka Nanase’s freestyle mastery. His analytical nature and sporadic egotistical outbursts solidified his role as the group's conscience, injecting structured logic into the team dynamic alongside Haruka’s water intuition, Makoto Tachibana’s nurturing presence, and Nagisa Hazuki’s infectious zeal.

Gradually, Rei cultivated authentic enthusiasm for swimming, transitioning from technical curiosity to emotional commitment toward both the sport and team bonds. His personal evolution unfolded through club engagements, where he harmonized scholastic rigor with athletic pursuit. Physically, he exhibits deep blue hair, purple eyes, and a distinctive facial structure and color scheme. Rei’s shifting position within the team echoes the narrative’s prioritization of interpersonal connections and self-development over competitive accolades.