Yūna Takashima stands among the earliest Heroes documented in the Hero Annals, fighting during the first Vertex invasion in 2018. She operated on the initial Hero team with Wakaba Nogi, Tamako Doi, Anzu Iyojima, and Chikage Kori. Cheerful and energetic, she acted as her team's moodmaker, sensitive to others' feelings and aware of her surroundings. She took pride in her role combating the apocalyptic threat, though her personal life remains largely mysterious.

During combat, the deaths of Tamako and Anzu triggered intense fury in Yūna. This event contributed to Chikage Kori's subsequent psychological deterioration. Injured in the same battle that claimed her teammates, Yūna underwent physical therapy. Her unwavering determination and perseverance through adversity deeply inspired Chikage, who admired her resilience despite being unable to express it directly. Yūna Takashima ultimately died fighting the Vertex, an event recorded in the Hero Annals.

Within *Yūki Yūna is a Hero: Churutto!*, a non-canonical setting where Heroes from multiple eras interact, Yūna appears alongside other characters. Her established traits of energy and enthusiasm persist in this spin-off, where she participates in group activities like the Hero Club's udon development endeavors. These lighthearted scenarios show her interacting with fellow Heroes across generations in a relaxed environment distinct from her original timeline's wartime context.