Dia Kurosawa serves as student council president during her third year at Uranohoshi Girls' Academy. She belongs to the influential Kurosawa family, known for managing Numazu's fishing industry alongside diverse enterprises spanning real estate, entertainment, human resources, and restaurants. Dia maintains a deeply protective relationship with her younger sister Ruby, whose shy and clumsy nature contrasts with Dia's own evolution from a timid childhood demeanor to her current serious and refined personality. Her closest childhood friends are Kanan Matsuura and Mari Ohara, with whom she briefly formed an early version of the school idol group Aqours in their first year. This unit dissolved abruptly when Mari sustained a leg injury pre-debut, prompting Dia to suppress her idol enthusiasm and initially resist Chika Takami's efforts to revive Uranohoshi's idol activities.

Dia exhibits a strict, formal, and perfectionist character, communicating in polite speech marked by "-san" honorifics and the signature phrase-ending "-desu wa." She corrects errors with her trademark "Bubbū desu wa!" and displays passionate, melodramatic reactions to idol topics despite attempts to conceal this interest. Her tsundere traits juxtapose a stern exterior with underlying warmth toward friends and groupmates. When lying, she unconsciously scratches a distinctive beauty mark beneath her right lip—a tell observed by Kanan and Mari. Her leisure pursuits include film-watching, reading, playing the traditional yamatogoto, and practicing kitsuke (kimono-wearing).

After opposing Chika's group, Dia joins Aqours and reveals her past as a μ's fan and her failed unit with Kanan and Mari. She names the new group "Aqours." Though initially struggling to bond informally with younger members, she gradually earns their trust and casual address. As part of subunit AZALEA, her image color is red. The anime concludes with her graduation following Uranohoshi's closure.

In *Over the Rainbow*, Dia travels post-graduation to Italy with Kanan and Mari. Their temporary separation from Aqours leads the others to pursue them overseas, where Dia supports Mari in confronting her disapproving mother—who attempts to enforce an arranged marriage. After Aqours' successful Rome performance changes Mari's mother's stance, Dia returns to Japan for the group's final live show at their dilapidated new school. Performing as a full nine-member unit, she exits with Kanan and Mari after the first half, symbolizing their graduation. A post-credits scene implies future idols will follow Aqours' legacy.