Realm of In Between

The Realm of in Between is the third plane of existence in the Kingdom Hearts series, and lies between the Realm of Light and Realm of Darkness.

Thus far the only known Worlds to exist in this plane are: Castle Oblivion, Twilight Town and The World that Never Was. Each world tied closer to the Realm of Light or the Realm of Darkness, with the exception of Twilight Town, which exists exactly in the middle.

Yen Sid's Tower also exists in this Realm.

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.