Darkness
Riku was always scared of the darkness, even when he claimed not to be afraid of it.
After the events, which resulted in the destruction of Destiny Islands, he lost himself completely to the darkness and became obsessed with bringing darkness to the worlds. When Ansem possessed him in the events leading up to the encounter at Hollow Bastion with Sora, he lost all semblance of the being that was Riku, and instead became a pawn of the darkness.
After Ansem perished when Sora opened the Door to Light, he still had a remnant of Ansem within himself. When he arrived at Castle Oblivion, he had to contend with Organization XIII members Lexaeus, Vexen, and Zexion , as well as contend with his inner darkness, which at times, threatened to take over him completely, but with the help of Mickey, he managed to keep his inner darkness at bay for a while.
When DiZ sent him to retrieve Roxas so that DiZ could combine Roxas with Sora so that Sora would be whole again. But when he went to The World That Never Was and confronted Roxas, he found himself transformed into Ansem once more. He stayed that way until DiZ died when his machine blew up. After that, Riku lost any traces of Ansem that were still inside him. It was only now that Riku realized the darkness was something fear, even if you accept it willingly.