Hikaru is the elder 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. Hikaru's feelings seem to be more passionate than Kaoru's, but the observant Kaoru realizes both twins' feelings first. Kaoru confesses to Haruhi in such a way that he'll be rejected, effectively sacrificing his own feelings for Hikaru's. This angers Hikaru, who wants Kaoru to be happy as much as Kaoru wants him to be happy. Hikaru says that they both need to go for Haruhi...although after the confession, Kaoru mostly just encourages Hikaru and gives him advice.
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.
Although he's the elder twin, Hikaru is not the more mature. He's very impulsive, and has an occasional cruel streak, and tends to not understand his own feelings very well; it took him a long time to figure out his feelings for Haruhi. Hikaru has also developed a friendship with Mori, to whom he goes for advice.