Kodachi, known as "The Black Rose" in the gymnastics community, is presented as Tatewaki Kuno's sister and a student at St. Hebereke School for Girls (St. Bacchus School for Girls in the manga). This is in contrast to several members of the regular cast, who attend Furinkan's mixed gender assemblage. As a master of "Martial Arts Rhythmic Gymnastics" and captain of the school's team, she often preemptively attacks her opponents with various weapons, including swords or a giant mallet to cripple them, thus winning by default. She calls it "fighting in all fairness before the match." Kodachi had seldom, if ever, lost a match before battling Ranma, a result of this practice. When these failed, she has also resorted to the use of more devious methods, once attempting to shake her foe's hand with thumbtacks between her fingers. She is especially fond of using paralysis gas and sleeping powders on people, even when she has no apparent motivation to do so.

Kodachi becomes enamoured with male Ranma after the latter acts as her 'knight in shining armour' by saving her from falling off the roof of the Tendo Dojo. Therefore, she is the only one of his suitors without some official or semi-official claim to him. Despite this, she considers him her lover and wants to make him her own through any means necessary. She and her brother have 'opposite' perspectives regarding this matter, since he despises Ranma's male form due to the latter's engagement to Akane and for 'enslaving' the mysterious 'pigtailed girl.' Correspondingly, Kodachi dislikes Ranma's female form, whom she believes to be a romantic rival. Regardless of his dislike for Ranma, Tatewaki has enthusiastically given them his blessing to go steady since this would free Akane for himself. He secretly imagines that they would have a miserable life together.