where disaster reigns
Riku, in Japanese, means land/shore or agony of separation. Let's take the first meaning. Riku loves sitting by the shore and looking out to the sea. Imagine the water coming up onto the sand. The water can easily alter the shape of the sand, similar to the way Maleficent and Ansem manipulated Riku. This may be stretching it, but the water also makes the sand on the shore darker, which relates to the darkness in Riku's heart.
Agony of separation is simple enough to understand. Riku goes through tremendous pain to try and save Kairi. He sacrifices himself to the darkness. In the end, he is separated from her once again. Although it could just be his separation from his friends, both Sora and Kairi.