Background:

Dia is a third-year student who is the president of the student council, and is the older sister of Ruby Kurosawa, who she shares a strong bond with. She is from an old family involved in fishing management whose name is well-known around the area. The Kurosawa family also owns a number of companies including real estate, entertainment, human resources and restaurant management. Dia is childhood friends with Kanan Matsuura and Mari Ohara.

In the anime, Dia initially rejects the idea of an idol club because of Chika Takami's incompetence and inexperience, chastising her for her lack of idol knowledge. It is later revealed in Season 1 Episode 4 that she used to be a fan of school idols, sharing this hobby with her sister Ruby, although something caused her to lose that interest. This is explained in Season 1 Episodes 6, 8 and 9, as she was part of a school idol group formed by Kanan, alongside her and Mari. However, Mari had suffered an injury right before their performance, so Kanan and Dia decided to step down and not perform. This failure caused Dia to hide her interest in school idols and initially try and prevent Chika's idol club from happening, but she later joins Chika's group in Episode 9 after Mari and Kanan reconcile.

Personality:

Dia is a very refined, polite and mature young woman who speaks in a formal style of Japanese, regarding all of her friends with politeness by referring to each of them as "-san", and usually ending her phrases with "-desu wa." Strict, hardworking, and a bit of a perfectionist, she dislikes it when things are done sloppily or incorrectly, which is part of the reason why she initially rejected Chika's application for a school idol club. She is very meticulous regarding small details, as far as going on tirades regarding the unorthodox reading of certain Kanji.

Though always trying to remain dignified and serious, Dia has a short temper, and she can be quite brash and melodramatic, while she also has a smiley heart and is very kind to her group. Whenever she denies something, she uses her catchphrase "Buu buu desu wa!" (ブッブーですわ！ "Bzzt bzzt, it is not!"), as if imitating a buzzer. She also tends to become overly zealous when school idols are involved, as she is very passionate about the subject, which becomes apparent in Season 1 Episode 2.

Because Dia can be quite harsh, she initially had the most difficulty in getting along with the rest of Aqours with the exception of her fellow third years, despite being adaptable like Ruby (as seen in Season 2 Episode 2); this is especially notable when referring to the second years. Although she is formal and polite, she also wishes the rest of the group referred to her as a friend by using more informal pronouns much like the other third years, despite referring to them with politeness. After Season 2 Episode 4, she becomes more patient towards them, and the group refers to her informally more often, displaying more closeness to them.

Dia can be described as a tsundere. While she is strict with a tough exterior, Dia is genuinely kind deep down and cares for all of her friends, particularly her younger sister Ruby, whom she is very protective and nurturing of. She wants nothing less than total success for Aqours, and constantly pushes herself and her friends to be the best they can possibly be in order to achieve their goals. After the events of Season 1 Episodes 7 and 8, she expresses the group her support for them, revealing part of the reason why she initially rejected the application for their school idol group is that she was afraid they would be demoralized in the same fashion she was in the past, demonstrating a soft side to her. Season 1 Episode 9 also reveals that, due to her close friendship with her fellow third years, she can have difficulty at settling down a harsh discussion, as she becomes insecure at which side to pick.

According to Mari and Kanan, Dia has a habit of scratching her beauty mark whenever she is telling a lie. Flashbacks of her younger self on certain episodes also reveal she used to be much like her sister Ruby, fairly timid, fearful and easily moved to tears, though she did start to develop her more serious side as time passed.

Clubs and Hobbies:

She is Uranohoshi Girls' High School's Student Council President. She is skilled in playing the Yamatogoto, and Kitsuke, the art of wearing a kimono. Her hobbies are watching movies and reading. She is also revealed to be an avid fan of school idols, namely μ's, although she refused to admit it at first.

Appearance:

Dia has a slim figure. She has long black hair that she styles in straight bangs on each side and has turquoise eyes. She has a beauty mark on the lower right side of her mouth.