Rio Amakusa, a petite 16-year-old with chest-length blonde hair and hazel eyes, dresses in her school uniform—a navy blazer over a gray sweater, light blue button-up shirt, and emerald skirt. As Ryo Amakusa’s younger sister and Ichika Arima’s closest confidante, she navigates familial tensions while fostering their deepening bond. Childhood exposure to Ryo and their father Zenji’s volatile arguments bred indifference in Rio, who often drowned out clashes with music. This detachment dissolved when Ryo pursued Ichika, as their shared connection to her became a bridge for sibling reconciliation. Rio transitioned from aloofness to active support, aiding Ryo’s courtship yet fiercely safeguarding Ichika, vowing consequences if he ever hurt her. Quietly perceptive, Rio masks sharp emotional intuition beneath a relaxed exterior. She detects Ichika’s insecurities with precision, dismantling her doubts by reframing Ryo’s admiration as rooted in her authenticity rather than perfection. Her pragmatic guidance steers their relationship toward mutual understanding. Strategically instrumental in key moments, Rio enabled Ryo’s initial outreach by discreetly sharing Ichika’s contact details. She later nudged Ichika to address Ryo’s excessive gift-giving, pushing them toward direct communication. While fostering their bond, her loyalty to Ichika remains uncompromising, tempering support for Ryo with clear boundaries. Zenji’s controlling tendencies and fractured history with Ryo initially distanced Rio from family entanglements, but Ichika’s presence slowly repaired these rifts. Her renewed kinship with Ryo reflects nuanced empathy—acknowledging his growth while critiquing past behavior. Academically and athletically skilled like her brother, Rio contrasts his extroversion with reserved stoicism. Rare emotional displays surface only around Ichika or Ryo, revealing protective instincts or dry amusement. Proficient in cooking and household management, her self-reliance and nurturing practicality further define her character.