Dia Kurosawa is a central character, serving as a third-year student and the student council president at Uranohoshi Girls' High School. She is the elder sister of Ruby Kurosawa and comes from a distinguished, long-established family in the local area known for its involvement in fishing management and various other business ventures. This upbringing has instilled in her a strong sense of pride, duty, and perfectionism, as she is expected to eventually take over family responsibilities.

Personality-wise, Dia is characterized by a stark contrast between her outward demeanor and her inner feelings, often fitting the description of a tsundere. She presents herself as a strict, refined, and mature young woman who speaks in a very formal and polite manner, typically ending her sentences with the catchphrase desuwa and correcting others with a buzzer-like Buu buu desu wa!. She is a hardworking perfectionist who dislikes sloppy or incorrect actions, which initially makes her seem harsh and unapproachable. However, beneath this serious exterior lies a genuinely kind and caring individual, especially towards her younger sister, Ruby, whom she is fiercely protective and nurturing of. Despite her best efforts to remain dignified, Dia has a short temper and can become quite dramatic and overly zealous, particularly when discussing school idols, a subject she is secretly very passionate about.

The initial motivation for Dia joining the school idol group Aqours is presented as being a result of losing a bet to Chika Takami and because her younger sister Ruby was being held "hostage". However, the truth is far more complex. In the past, Dia was once a huge fan of the idol group μ's and, alongside her childhood friends Kanan Matsuura and Mari Ohara, formed the original Aqours. A traumatic incident where Mari was injured before their first performance led the group to disband, causing Dia to bury her love for school idols out of guilt and a fear of failure. Consequently, she initially opposes Chika's efforts to start a new idol club, chastising her for her lack of knowledge and experience. Her strict opposition is eventually overcome when her past misunderstandings with Kanan and Mari are resolved, allowing her to finally reconcile with her passion and join the new Aqours as a core member, even providing the group with its name.

Throughout the story, Dia plays a crucial leadership role within Aqours. Her perfectionism and experience make her the natural choice for directing the group's dance routines and overseeing their rigorous training schedule. She is a relentless believer in the group's potential for success, refusing to accept anything less than total victory at the Love Live! competition. Her key relationships are central to her development. Her bond with her sister Ruby shows her soft, protective side, while her rekindled friendship with Kanan and Mari allows her to let go of past regrets. Initially, her formal and strict nature causes friction with the younger members like Chika, but as the series progresses, she learns to be more patient and the entire group grows closer, often addressing her less formally as a sign of friendship.

Dia's character development is marked by her transformation from a girl bound by the fear of past failure and a rigid sense of propriety to a passionate and supportive leader who learns to embrace both her duties and her desires. She evolves from a perfectionist who tries to hide her true interests to a dedicated team player who fights for a collective dream. Her notable abilities extend beyond idol activities; she is highly proficient in traditional Japanese arts such as playing the yamatogoto (a stringed instrument), singing, and the art of kimono dressing. She also has a habit of scratching the beauty mark near her mouth whenever she is telling a lie, a small tell noticed by her closest friends.