Cooan is the youngest of the sisters (oldest in the dub) with the attack called "Dark Fire" ("Ice Fire" in the dub), and she represents a counterpart to Rei Hino. Her name is derived from kouankou, the Japanese word for kermesite.
In the manga, Cooan informs Rubeus about sensing someone akin to fire who could potentially join the Black Moon. Rubeus assigns her "Code001, Operation Re-cruit" to convert humans to their cause. Posing as a student named Kurozaki Cooan at T*A Girl's Academy where Rei Hino studies, she becomes the leader of a paranormal club called the Black Moon. The leader of another supernatural club on campus is disturbed by how cold her hands and eyes are, and a black saucer-shaped object flies away after she leaves. Rei later has a nightmare about Cooan using her powers to burn the body of a child. At the school's culture festival, Cooan sets up a booth telling fortunes of how and when her clients will die, but when Rei appears, she tells her that everyone must die, though those who follow the Black Moon will die better. After Rei uncovers droids disguised as nuns, she transforms into Sailor Mars and defeats them. At this point, Cooan reveals herself. In the ensuing battle, she blasts Sailor Mars with fire, and Sailor Moon and Tuxedo Mask kill her using the Cutie Moon Rod. Mars, still trapped in Cooan's flames, is then abducted by Rubeus.
In the anime, Cooan is very vain about her appearance, swearing death upon anyone who smudges her makeup or ruins her hair. She relies on her charms to get what she wants and believes she and Rubeus are in a relationship, competing with her sisters for his love. However, when she gets him a gift of cologne, he cruelly crushes it in front of her and tells her that her next mission will be her last if she fails him. While surveying the Hikawa Shrine for Chibiusa's appearance, she expresses disdain for Rei and Yuuichiro's relationship, unfavorably comparing it to her perceived strong bond with Rubeus. Later, she gets caught up in a discussion with Rei and Usagi, disagreeing with Rei's belief that the most important thing in a relationship is believing in and loving the other person, claiming it only matters how much the other person loves you. Her beliefs are challenged when Chibiusa arrives and Cooan tries to kill her. Yuuichiro and Rei (as Sailor Mars) both get seriously hurt trying to protect each other, surprising Cooan given her previous perceptions of them. She ultimately fails to kill Chibiusa, and Rubeus appears to evaluate her. Begging for his help, believing their love will be enough, he tells her he has no use for her anymore and never loved her. He gives her a time-bomb and tells her to kill herself and the Sailor Senshi before abandoning her. Before she can use it, the other Sailor Senshi arrive and knock the bomb away, causing it to explode harmlessly in the sky. Cooan starts attacking everyone in a blind rage, feeling she has nowhere left to go, but Sailor Mars, after getting hurt protecting Cooan from the other Senshi, convinces her that she can have a second chance. Sailor Moon then purifies her with the Silver Crystal, turning her into a normal human woman. She later talks her next oldest sister, Berthier, out of killing herself and the other Senshi after Berthier is also tossed aside by her sisters and Rubeus, and later convinces Petz and Calaveras that they too are being used.
