The younger, slightly more honest, arguably more mature of the mischievous Hitachiin twins.
Kaoru is the younger of the Hitachiin twins. Prior to entering high school and the host club, the two were tired of no one being able to tell the difference between them-- or worse, randomly deciding on a preference, such as when a girl would give a love letter to one twin. That twin would then say that his brother wanted to date her; the girl generally would respond that this was okay to her, and the twins would then be cruel to her because she didn't seem to care which was which. Then they met Tamaki, who was never able to meet their challenge of telling them apart but was able to convince them to join anyway.
As the manga starts out, even the reader generally can't tell Hikaru and Kaoru apart. The only person who can tell them apart is Haruhi Fujioka; when asked to explain she simply says that Hikaru is "more mean" than Kaoru. At first they're disturbed at what seems to be an intrusion into their two-person world, but over time both twins come to care for Haruhi very much, pretending to "adopt" her and eventually both crushing on her. Kaoru realizes his feelings-- and Hikaru's --first. In spite of being the younger twin he is generally considered to be the more mature twin, and he is torn about whether to reach for his own happiness or for his brother's. In the end he confesses to Haruhi and is turned down, but he arranged it intentionally so that his brother could try for her. Hikaru, however, rejects this notion and tells Kaoru that he should go for her sincerely.
In the backdrop of Hikaru and Kaoru's discussions over Haruhi is the fact that while the two want to remain close, they know that they have to take their own steps independent of one another someday. In recent chapters, Hikaru dyed his hair brown and is therefore distinguishable from Kaoru.
Both twins are mischievous, prank-loving trouble makers who enjoy their time in the Host Club because of the way they get to manipulate the girls. Their "brotherly love" act is more or less a game to them. Kaoru is the more observant of the twins, and is more sensitive to the feelings of others. He realizes his and Hikaru's feelings before Hikaru does, and analyzes the situation fairly deftly: Tamaki likes Haruhi, Hikaru likes Haruhi, he likes Haruhi, and Haruhi herself is blissfully unaware. However, his observations may lead him to overthink the situation. He doesn't think about what how his brother will feel if he throws away his chances with Haruhi, thinking that only he can work this out on his own. In spite of his position as younger sibling, he is often the one who helps guide Hikaru...though he's perfectly willing to prank him on occasion as well.
In recent manga chapters, Kaoru has stuck a friendship with Honey, to whom he goes for advice.