Riku loves exploring unknown and possibly dangerous places. His curiousity is boundless, and is further amplified when Kairi arrives on the Destiny Islands from another world. He begins to ponder what lies beyond the world they live in. Though he may be pensive, he's far from the cold, distant type. As mentioned earlier, he pokes fun at his friends, especially Sora, to show that he cares about them.
Riku is the quite likely the most selfless person in the game. In the first installment, he lets himself be consumed by darkness in order to save Kairi. In the next major one, he gives up his physical form so he can continue using dark powers, all in an effort to help Sora. If risking eternal solitude in darkness doesn't qualify you for being selfless, I don't know what does. As long as his friends are safe, he doesn't care what happens to himself. He's the type of person that would die without any regrets to save a loved one.