Ikuya Kirishima, younger brother of former Iwatobi Junior High School Swim Club captain Natsuya Kirishima, was born on March 3rd. Though he initially enjoyed swimming as a child, he grew disenchanted with the sport due to his complex relationship with his older brother. Ikuya idolized Natsuya, who emotionally distanced himself in a misguided attempt to foster Ikuya's independence. Perceiving this as abandonment rather than support, Ikuya renounced their shared interests, including swimming. During junior high at Iwatobi, Ikuya resisted joining the swim club because Natsuya was captain. He was eventually persuaded by Haruka Nanase, Makoto Tachibana, and Asahi Shiina to participate. His notable talent in breaststroke emerged, but his perfectionism drove dangerous overtraining habits. These led to oxygen deprivation during practice, requiring hospitalization multiple times and once necessitating rescue by Haruka. Initially distrustful and cynical toward teammates, Ikuya gradually warmed to them and valued the team's camaraderie. He hoped to continue swimming with them, but these plans dissolved when Haruka quit competitive swimming after a falling-out with Rin Matsuoka. Makoto's withdrawal of support for Haruka and Asahi's relocation further shattered the team, leaving Ikuya feeling abandoned. Following this disappointment, Ikuya moved to America with Natsuya. Attending junior high there, he met Hiyori Tono, who became his closest friend and primary supporter. Ikuya later returned to Japan for high school, enrolling at Shionezaki High School with Hiyori. He subsequently studied at Shimogami University, where both joined the swim team. During this period, he mastered all four swimming strokes and specialized in the 400-meter individual medley (IM), with his freestyle technique noted for its similarity to Haruka's. Ikuya's personality exhibits quiet intensity and a deep-seated fear of mediocrity, defined by his desire to "never be ordinary." Fragile self-esteem and health complications—including near-drowning incidents and residual trauma from overtraining—shaped his adversarial view of water as an "enemy" to conquer, contrasting sharply with Haruka's harmonious relationship with it. His reliance on Hiyori to monitor his health during Natsuya's global swimming career strained their friendship, as Ikuya remained oblivious to Hiyori's burdens until later confrontations. His development culminates during university when he reconnects with former junior high teammates. After years of resenting Haruka for breaking their promise to swim together, Ikuya accepts Haruka's apology. Their long-delayed IM race symbolizes Ikuya's emotional reconciliation, helping him mend relationships with Natsuya, Hiyori, and himself. He begins to see himself as "humanized," though competition and winning remain core motivations. The Hans Christian Andersen fairy tale *The Little Mermaid* serves as a recurring metaphor for his struggles; Ikuya relates to Ariel's physical pain on land, interpreting it as a necessary sacrifice to transcend limitations. Physically, Ikuya has dark teal hair, amber eyes, and a lean, muscular build. He grows from 143 cm (4'8") in middle school to 176 cm (5'9") by university. His serious demeanor is reflected in his appearance; his competition attire typically consists of black jammers featuring subtle brown and gray designs. In later competitive arcs, Ikuya participates in national and global tournaments. He swims in a four-member relay alongside Rin Matsuoka, Haruka Nanase, and Sosuke Yamazaki at the Fukuoka Tournament, securing first place. This achievement underscores his integration into a renewed team dynamic, marking his progression from an isolated perfectionist to a collaborative athlete.