There are 4 powers.
Light, Darkness, Twilight and Void/Nothing.
We don't know who the light is in terms of monsters, but Sora has taken the path of Light.
Darkness is the Heartless and we know that Mickey and Ansem (Xehanort) have taken this path.
Twilight is the road that is taken by DiZ and Riku. We don't know what kind of creature has the powers of twilight, or if one even exists.
Nothingness is the "road" that is given to the Shells. They really don't get a choice. The Nobodies are stuck on this road as well. My guess is that you need a Heart to decide which power you want to follow (Light, Darkness or Twilight) and since Shells lack this, they can not follow a path.
Light, Darkness, Twilight and Void/Nothing.
We don't know who the light is in terms of monsters, but Sora has taken the path of Light.
Darkness is the Heartless and we know that Mickey and Ansem (Xehanort) have taken this path.
Twilight is the road that is taken by DiZ and Riku. We don't know what kind of creature has the powers of twilight, or if one even exists.
Nothingness is the "road" that is given to the Shells. They really don't get a choice. The Nobodies are stuck on this road as well. My guess is that you need a Heart to decide which power you want to follow (Light, Darkness or Twilight) and since Shells lack this, they can not follow a path.