Kaoru Kamiya is the young proprietor and acting instructor of the Kamiya Kasshin-ryū kenjutsu dojo in Tokyo. She inherited the small dojo and its fighting style from her late father, Kamiya Koshijirou, who was drafted into a police swordsmen unit and died defending a comrade during the Seinan War. At the beginning of the story, the dojo has no students due to an incident where a murderer claiming to be the Hitokiri Battōsai, the same title once held by the wanderer Himura Kenshin, damaged the school's reputation.

In personality, Kaoru is known for being strong-willed, obstinate, and fiercely independent. She carries the pride of her samurai lineage and is a force to be reckoned with, gladly putting her life on the line to protect her father's legacy and her friends. She has a fiery and violent temper, often displaying it without restraint when she feels slighted or insulted, and can be somewhat childish, sometimes prompting her to violence when she cannot come up with a witty verbal retort. Despite this, her most unique trait is her ability to quickly see the good in others, even when it is hidden deep beneath the surface, and she is always ready with a kind word of encouragement. She wholeheartedly believes in the Kamiya Kasshin-ryū's ideal of katsujin-ken, or "swords that give life," which teaches that swordsmanship should be used not to kill, but to protect and control an opponent. Although some may call her naïve for holding this belief in a more selfish era, her sense of duty and honor prevents her from giving up. While she has a masculine athleticism and is unapologetic about it, she also has a strong feminine side, appreciating fine clothes and art, and worrying that she is too plain to be attractive. She is notably possessive of Kenshin and becomes jealous when other women show interest in him, though she remains shy and uncertain about her true feelings for much of the series.

Kaoru's primary motivation is to uphold her father's legacy and the philosophy of the Kamiya Kasshin-ryū. She is deeply selfless, driven by a compulsion to help others and protect life. She opens her home to various people in need, including the former assassin Kenshin, the young pickpocket Myōjin Yahiko, the street fighter Sagara Sanosuke, and the doctor Takani Megumi, essentially forming a surrogate family. One of her greatest fears is that Kenshin's hitokiri nature will resurface, forcing him to return to his wandering life and leave her alone.

Central to her role in the story is her relationship with Himura Kenshin. After she is saved from a murderous impostor by the real Battōsai, she invites the wanderer to stay at her dojo, effectively ending his years of roaming. She becomes his primary motivating factor and the anchor to his shattered self-worth, serving as the one who must keep his violent past in check. Her unwavering belief in him and her presence help him find a measure of peace. She is also the acting instructor and a motherly figure to Yahiko, her first and most dedicated student, with whom she has a bickering yet deeply caring relationship resembling that of siblings. Her relationship with Megumi is initially marked by jealousy over Kenshin, but it evolves into a friendship between two strong-willed women. She also develops a close friendship with Sanosuke, who becomes a protector of the dojo.

Throughout the series, Kaoru demonstrates considerable development. She begins as a lonely dojo keeper struggling to maintain her father's legacy, but grows into a confident woman and a skilled swordswoman. When Kenshin leaves for Kyoto to face the deadly Shishio Makoto, she falls into a depression but ultimately follows him, showing her resolve to support him by his side. In a major story arc, she is kidnapped by Yukishiro Enishi, who leaves a replica of her dead body to torment Kenshin. Her survival and eventual rescue lead to her marriage to Kenshin, and she becomes the mother of his son, Kenji Himura. At the end of the manga, she cuts her long ponytail to adopt a more motherly demeanor.

Regarding her notable abilities, Kaoru is a highly skilled swordswoman. Although her combat skills are dwarfed by fighters of Kenshin or Sanosuke's caliber, the manga creator has stated that her skill is at a level equivalent to or superior to that of a national-level champion. She is proficient in the techniques of the Kamiya Kasshin-ryū, which focuses on subduing rather than killing, often using a bokken (wooden sword). Her techniques include the succession techniques of Hadome, a defensive move to catch and restrain an opponent's blade, and Hawatari, an offensive counter-attack to disarm them. She has also demonstrated the ability to use the Tsuka no Gedan: Hiza Hishigi, a technique where she uses the hilt of her sword to crush an opponent's kneecap. She is very analytical in battle, able to quickly deduce the mechanics of an opponent's technique, and is extremely agile. In contrast to her martial prowess, she is notably unskilled at domestic chores; her cooking is famously poor, a running joke throughout the series.