Kaoru Hitachiin is the younger identical twin brother of Hikaru Hitachiin, both members of the Ouran Academy Host Club. Born on June 9 under the Gemini star sign, he shares a close yet complex bond with his brother, fundamentally shaped by their early experiences.

During childhood, Kaoru and Hikaru remained inseparable and deeply distrustful of outsiders. A pivotal betrayal by a maid they admired cemented this: she exploited their "Which One Is Hikaru?" game, promising the safe combination for participation, only to steal family valuables. Her declaration that no one could ever tell them apart intensified their isolation and hindered their social growth. This fostered a persistent "us versus them" mentality, leading them to shun anyone who couldn't distinguish them and to use pranks as barriers.

Physically, Kaoru possesses short auburn hair, amber eyes, a thin face, and pale skin. He typically parts his hair to the left (appearing parted on the right when facing him), while Hikaru parts his to the right. Kaoru usually positions himself on Hikaru's left. Though they claim to switch parts daily, their hairstyles remain consistent unless deliberately confusing others. Kaoru experiments with colors like powder blue and pink to obscure identities, though Haruhi Fujioka consistently identifies him. His fashion sense favors outlandish designer clothing, usually distinct in style or color from Hikaru’s. Post-graduation, Kaoru grows his hair longer than his brother. In the Host Club, his designated rose color is orange, symbolizing happiness in Japanese culture and romantic enthusiasm in Western contexts.

Personality-wise, Kaoru exhibits greater maturity, level-headedness, and emotional perceptiveness than Hikaru. While both perform a mischievous "little devil" persona and a "forbidden brotherly love" act for clients, Kaoru displays more self-control and insight when separated from his twin. He recognizes early that their codependence must end for individual growth. Despite sharing romantic feelings for Haruhi Fujioka, Kaoru suppresses his own to support Hikaru’s pursuit, even orchestrating their date. His Gemini traits include adaptability and wit, alongside struggles with indecisiveness. He closely monitors Hikaru’s emotions, often mediating conflicts or apologizing on his behalf, as when Hikaru lashed out at Haruhi’s friend Arai.

Family dynamics further define Kaoru. His mother, Yuzuha Hitachiin, a renowned fashion designer, frequently confuses the twins for amusement, though she can distinguish them. His father, Hayato Hitachiin, manages a software company and consistently identifies them. Their grandmother, Kazuha Hitachiin, a free-spirited floral designer, shares a history of mutual pranks with the twins that later reveals respect for their artistic talent. The twins also have a younger sister, Ageha (manga-only), who rejects their gifts. Their parents’ work-related absenteeism entrenched the twins’ mutual reliance.

Kaoru drives the twins’ journey toward independence. He initiates efforts to develop separate identities, pursuing distinct academic interests—favoring English and modern Japanese literature while Hikaru prefers STEM subjects. In the manga, Hikaru dyes his hair darker to facilitate differentiation. Kaoru seeks advice from Mitsukuni "Honey" Haninozuka during conflicts, while Hikaru turns to Takashi "Mori" Morinozuka. Post-series, Kaoru becomes a theatrical costume designer. Both twins eventually move into separate rooms and establish independent social circles. Their bond endures, but they prioritize personal growth, with Kaoru supporting Hikaru’s romantic relationships even after Haruhi marries Tamaki Suoh.

Key differences from Hikaru include Kaoru’s selflessness versus Hikaru’s possessiveness, his calm optimism against Hikaru’s volatility, and his preference for classic clothing over Hikaru’s bold patterns. Shared traits encompass a love for Italian food, spicy dishes, maple syrup, and prankster tendencies.