Riku, who is the true keyblade master, lost his body over to Ansem after joining with him. Riku was than powerless, as he was being controlled by Ansem. In Kingdom Hearts, you face Riku twice as he is controlled by Ansem. Riku abandoned the body and wound up on the edge of the world of darkness, and was led by an unseen force.

In Sora's last battle (the one where you defeat Ansem), Riku is able to cross over to the "other side", as when after Ansem was defeated, the door had to be shut and Riku stepped forward and helped close it shutting himself onto the other side along with King Mickey.

Is Riku the blindfolded unknown who is shown in "Another Side, Another Story?"

Tetsuya Nomura has confirmed that one of the characters in "Another Side, Another Story" is Riku. In the video, there are only two unknowns that are shown and it said to be the the blindfolded unknown. Rumors have it that the blindfold is to cover his eyes due to the light exposed to him. Many people already come to the conclusion that the blindfolded unknown is Riku due to the very similar hair style as well.

In Chain of Memories, Riku is all over the place. The first indication of Riku we see is that in a screenshot in Monstro, a translation is reported as Geppetto saying to Sora and the gang, "You will meet a precious opponent, you once got seperated from." This is obviously Riku and but what is more interesting is how Geppeto knows this.

In certain magazine scans, we are able to see Riku (both normal Riku and dark Riku so we know his character changes in Chain of Memories). Below is a translation from the above magazine scan:

Normal Riku: 'Ask your memories about the reason. If you remember why Namine disappeared from the island, you should know.'
Dark Riku: 'Sora...if you go any farther, Namine will be hurt.'

The girl, whom one could see so far in some pictures, is called most likely Namine, is thus not not Kairi. This is the girl we see in Chain of Memories drawing the castle. In a scene Riku is to say that he will never let someone to her.

In another translation, we see Riku say the following: But I know something bad is happening. I'm worried about everyone else so I'm going to go find them! In essence, Sora is said saying "Riku!? Riku's here too?".

In the Chain of Memories trailer, we also see Mickey and Riku in black robes at the same croassing we have seen Sora at before. Apparently they meet with the Unknown in the red robe, which could be Ansem, and Riku had an unknown outfin on.

In the below screenshot, Sora says to Riku: I can't go back 'til I save you and Namine.

To recap: We know Riku is in Chain of Memories (duh), but what exactly is his role? Who knows. It seems the story is progressing around him as we are able to see Riku in his dark outfit and in his normal outfit as well as in an unknown cloak walking towards Ansem (which may start off as some sort of scene at the beginning of Chain of Memories). From the screenshots above, Riku is trying to protect the new girl, "Namine", who we barely know anything about. It seems that it'll all fall down on more translations and some pretty darn good interpretations to figure all this out.