Yoshiko Tsushima attends Uranohoshi Girls' High School as a first-year student, later transferring to Numazu High School after Uranohoshi's closure. Born in Numazu City's urban area, she went to the same kindergarten as Hanamaru Kunikida, whom she nicknames "Zuramaru." Her mother works as a teacher.

During kindergarten, Yoshiko adopted the persona of "Yohane," a self-proclaimed fallen angel cast from heaven for angering God with her perceived cuteness. This chuunibyou identity became entrenched in middle school, partly as a reaction to feeling ordinary. She insists on being called Yohane, rejecting her birth name, and incorporates dramatic speeches, devilish accessories, and a deliberately lower-pitched voice when in character, stemming from a desire to feel significant and distinct from mundane existence.

Associated with her fallen angel identity is extreme misfortune, which she interprets as a divine curse. This includes practical inconveniences like sudden rain during outings, frequent falls in snowy conditions, consistent lottery losses, and illnesses before exams. She often attributes group setbacks to this bad luck.

Despite her flamboyant persona, Yoshiko is fundamentally kind, thoughtful, and deeply values friendship. She affectionately refers to Aqours members as "little demons," singling out Riko Sakurauchi as "Little Demon Lili/Riri." Her compassion is evident when she helps Riko overcome a fear of dogs. Though her Yohane act sometimes causes social friction, she treasures Aqours' acceptance of this aspect of her identity. While her commitment to the persona makes normal social interaction difficult, she can engage conventionally when necessary.

Her interests and skills include playing video games (especially fighting and racing genres), performing magic tricks, video editing (serving as Aqours' primary editor), and live-streaming online fortune-telling sessions despite her bad luck. She favors "little devil" fashion aesthetics, enjoys extremely spicy food, chocolate, and strawberries, and dislikes oranges/mikan and coffee.

Physically, Yoshiko has navy blue ("midnight blue") hair styled in a high bun on her right side and pinkish-purple eyes. Her typical attire features dark colors, often black, reflecting her fallen angel motif, and she maintains a slim figure.

In *The School Idol Movie: Over the Rainbow*, Yoshiko travels with Aqours to Italy to locate the missing third-year members. Her Yohane identity becomes plot-relevant when the group discovers a clue from Mari written in Italian: "When Yohane looks upon the land that she protects, the fairies will lead the way." Yoshiko deduces "Yohane" refers to Saint Giovanni (the Italian equivalent of John), guiding them to Florence and aiding in reuniting with the third-years. This demonstrates the integration of her persona into the group's problem-solving.

Following their return to Japan, Yoshiko's middle-school acquaintances, ardent followers of her Yohane persona, assist Aqours in preparing for their final live performance, highlighting how her unique identity inspires others. The movie concludes with a post-credits scene hinting at new aspiring school idols, continuing the legacy Yoshiko helped build.

Her character development revolves around embracing her Yohane persona as an authentic part of herself. Initially, she attempted to abandon the act upon entering high school and considered leaving Aqours after her "fallen angel" concept caused performance complications. However, the group's reassurance solidified her self-acceptance. By the movie's events, she confidently utilizes her Yohane traits for the group's benefit, signifying growth from viewing her identity as a liability to an asset.