Yoko Hinageshi, Minato's 16-year-old younger sister, bears a name meaning "ocean child," anchoring her to the story's coastal setting. Her blunt, tactless nature earns Minato's comparison to a venomous blue-ringed octopus, which Hinako recognizes. Viewed as an annoying younger sibling by her parents—who contrast her behavior against Minato's responsibility—she once skipped school but stopped before his death.

After Minato's death, Yoko evolves. She confronts Hinako's grief head-on, rejecting her self-blame and urging her forward. Similarly, she challenges Wasabi Kawamura, Minato's firefighting colleague, to stop measuring himself against her brother and embrace his own identity. Her abrasive honesty consistently delivers accurate, constructive critiques.

Yoko harbors longstanding feelings for Wasabi, sparked when he defended her during her truancy, restoring her self-worth and motivating her return to school. Later, she confesses openly, crediting him for helping her endure bullying—marking her shift from defensiveness to vulnerability.

In the climax, Yoko confronts arsonists replicating Minato's last fire. While pursuing evidence, she and Hinako become trapped in the burning building. Overwhelmed by fear—breaking her usual stoicism—she sobs in Hinako's lap, demonstrating her growth. Minato's spirit intervenes, enabling their escape by riding a wave to safety.

One year later, Yoko and Wasabi are dating. Hinako deliberately grants them privacy, signaling Yoko's integration into a supportive circle. Wasabi's newfound confidence and Yoko's softened demeanor reflect their mutual positive influence.