Hiyori Toono, a dedicated backstroke swimmer born January 20, grew up in a household of working parents, fostering self-reliance but leaving him a shy child who formed an early bond with a girl named Tomo during solitary sandbox play. His family’s relocation to the United States led him to meet Ikuya Kirishima in high school, sparking a connection that endured through their enrollment at Shionezaki High School and later Shimogami University as teammates. A pivotal moment solidified their bond when Hiyori rescued Ikuya from a near-drowning caused by oxygen deprivation, prompting Ikuya’s brother Natsuya to entrust Hiyori with his care. This responsibility fueled Hiyori’s overprotectiveness, particularly toward Ikuya’s middle school friends from the Iwatobi Swim Club, whom he viewed as detrimental to Ikuya’s emotional health and career. His confrontations with Haruka Nanase, Makoto Tachibana, and Asahi Shiina escalated into a swimming challenge against Makoto—a race Hiyori won, though his stance softened only after Ikuya reconciled with his past. This shift culminated in Hiyori joining Ikuya on Shimogami University’s relay team at the All-Japan Invitational, where Ikuya’s victory over Natsuya repaired their sibling relationship. Natsuya’s subsequent apology allowed Hiyori to relax his vigilance and engage more openly with others. Hiyori’s demeanor fluctuates between gentle support for Ikuya—often veiled in cheerful compliments that obscure his own feelings—and sharp criticism toward outsiders. He redirects focus to Ikuya’s achievements during personal conversations, suggesting buried insecurities. Visual motifs, such as a fleeting childhood image of Ikuya reflected in Hiyori’s eye during an opening sequence, imply a deeper, possibly unacknowledged connection. This theory gains traction through Hiyori’s visceral reactions to Ikuya’s forgotten memories, like a phone left behind at a restaurant. An enthusiast of swimmer statistics, Hiyori’s studious appearance features glasses and a polar bear motif. His strategic intellect emerges in courteous yet calculated interactions, disarming adversaries with calm smiles during conflicts. Though initially framed as antagonistic, his actions arise from fiercely protective loyalty, reflecting broader relational patterns within the series. Narrative cues and emotional undercurrents hint at unresolved complexities in his bond with Ikuya, woven through their shared history and unspoken past.