Realm of In Between

The Realm of in Between is the third plane in the Kingdom Hearts series and lies between the Realm of Light and Realm of Darkness. Thus far the Worlds in this plane are Castle Oblivion, Twilight Town and The World that Never Was arranged like so: Light < b < a > c > Darkness

Riku's Keyblade
Because Riku is able to live in the light and yet accept darkness, it is possible that his Keyblade, Way to the Dawn which serves as an entity of both light and darkness, chose him to be its master because of this reason.