Unmarked spoilers, beware.

Riku was hardly evil. He delved into darkness to save Kairi. He was so determined to help her that he sacrificed everything, eventually even his body, to the darkness. His rivalry with Sora only made him work harder, especially towards the end, to be the stronger of the two.

He was believable as well, as most of the characters in Kingdom Hearts were. Well, really only Riku and Sora were shown enough to grow as characters, but SE did a good job showing both their growth.

Honestly, Riku was my favorite character in Kingdom Hearts, and I hope he gets a larger role in Kingdom Hearts II as one of the unknowns. He didn't really shine through in Kingdom Hearts until the end. I would have liked to have seen a bit more of his side of the story.