Dia Kurosawa, third-year student council president at Uranohoshi Girls’ Academy, carries herself with the refined poise of her upbringing in a family renowned for managing fisheries, real estate ventures, entertainment venues, and hospitality services. Her formal speech, punctuated by honorifics like "-san" and the corrective phrase "Būbū desu wa!", reflects a life shaped by tradition and meticulous standards. Once resistant to the revival of school idols, she harbored unresolved guilt from her failed attempt to form a group with childhood friends Kanan Matsuura and Mari Ohara—a project abandoned when Kanan fabricated stage fright to shield Mari from worsening an injury. This past failure fueled Dia’s initial skepticism toward Chika Takami’s fledgling idol club, fearing history might repeat. Yet witnessing Aqours’ resolve, coupled with mending ties to Kanan and Mari, reignited her passion, leading her to join under their original group’s name.

Beneath her stern exterior lies a fiercely protective sister, wary of exposing Ruby to the heartbreaks she once endured. Though initially discouraging Ruby’s idol pursuits, Dia gradually embraces her sister’s dedication, softening into a reluctant yet devoted supporter. Her tsundere nature surfaces in covert acts of care, such as procuring a backup generator for Aqours’ debut concert. A connoisseur of school idol history, she harbors deep admiration for μ’s leader Eli Ayase—a fandom that once clashed with her guarded stance toward Aqours, mirroring her internal struggle between duty and desire.

Proficient in traditional arts like yamatogoto and kimono dressing, Dia balances her structured life with quiet hobbies: losing herself in books, films, and matcha-flavored indulgences. Her elegant appearance—long ebony hair, turquoise eyes, and a signature beauty mark—contrasts Ruby’s youthful charm, embodying maturity. In the fantastical realm of *Yohane the Parhelion*, she transforms into Scarlet Delta, a tokusatsu-inspired heroine channeling her voice actress’s genre roots.

Dia’s journey traces her evolution from a perfectionist bound by past regrets to a leader who champions collective triumph over solitary ideals. Through reconciliation and camaraderie, she learns to harmonize familial legacy with personal aspirations, embodying resilience forged through shared struggle and unwavering support.