riku's childhood
Not too much about Riku's childhood was revealed in the game. Only a few scenes were shown, when Riku was small. However, the way he acted in those scenes can tell you a lot.
He seems to act serious... and almost like he is trying to act a little older than the others, however, he still goes into the Secret Cave with Sora... showing that he is still young and adventurous.
In the scene when Sora is in the Secret Cave, talking about a monster, he seems to be intent on finding that monster, even though it is nonexistant. Riku, however, continually tells Sora that there is no monster, and that it is only the wind. He didn't believe in monsters, etc... which shows that he was a lot more mature than the others.
His more mature outlook on those sort of things stayed with him, and he grew to become the big brother of all the Islanders, looking out for them, and taking care of them. It showed that he grew up thinking it was his job to care for the younger children, which is probably why he felt that he didn't belong with Sora in Traverse Town, seeing as Sora could take care of himself, and didn't need the guidance of the older boy.