Personality

He is usually the one that leads his brother into the different kinds of trouble-making they come up with. His words and actions are usually more menacing than his brother's. He is loud and particularly crazy about schemes and mischief. He also doesn't care if he's being rude or obnoxious, especially if he thinks he's in the right.

Though he may act tough and uncaring, his one weakness is his brother. He cares for him deeply, and if he's hurt, he gets upset easily, though he won't show it to anyone but Kaoru. He gets angrier a lot easier than his brother. Kaoru usually tones him down just a bit.

Hikaru doesn't have many people that matter to him. He is codependent on Kaoru, and tends to shut out everyone else. He is especially immature and acts out his feelings without restraint. He only knows how to throw his feelings around rather than expressing them calmly, so he looks like a possessive kid. He doesn't know how to go about showing his genuine feelings. Hikaru still has to learn the basics of personal interaction.

History

Hikaru Hitachiin is a first-year student at Ouran Academy, and the older of the Hitachiin twins. (History is explained throughout the anime/manga, so I don't really think I need to delve into this any further)