Riku is Sora’s friend and another person that wields a Keyblade but the only Keyblade Master and is likely to play a big part in the story for Kingdom Hearts 3.
As Sora is not going to be the only playable character in KH3 it is likely that one of the other characters you’ll be able to play as is Riku. In the story King Mickey and Riku are on a mission to save Aqua, hence the chance of him being playable.