Yūko Hinageshi is Minato Hinageshi's younger sister and a high school student working part-time at a café. Minato nicknames her "blue-ringed octopus," reflecting her blunt communication style and protective nature. She lives with Minato before his death, observing his relationship with Hinako Mukaimizu firsthand.

After Minato drowns, Yūko interacts with Hinako during a visit to the Hinageshi home. She reveals Minato was inspired to become a firefighter because he was saved from drowning as a child and discloses that Hinako was his rescuer. Yūko shares a drafted, unsent text message from Minato's phone containing his final encouragement for Hinako to "ride her own wave."

Yūko tracks the group responsible for the fireworks that caused Minato's apartment fire. She enlists Hinako to gather evidence of their plans for another display at an abandoned building. Both become trapped when the building ignites. Yūko sustains injuries but is rescued after Hinako summons Minato's spirit, creating a wave that extinguishes the fire. Placed on a backboard by Hinako, they ride the wave down the building to safety.

Her character arc includes a romantic subplot with Wasabi Kawamura, Minato's firefighting colleague. Yūko confesses her feelings to Wasabi, reminding him he once inspired her to return to school after bullying. By the following Christmas, Yūko and Wasabi are in a relationship. They join Hinako to celebrate her lifeguard certification at Chiba Port Tower, maintaining Yūko's connection to both characters after Minato's passing.