The Realm of in Between is an area in Kingdom Hearts II only accessible during the final chapter in Twilight Town as a rift/portal between Twilight Town and The World That Never Was.
It is only seen two times throughout Kingdom Hearts II; when Kairi and Pluto escape from Axel at the Destiny Islands, and when found in aforementioned Twilight Town. It is only accessible after guessing the password of the lab computer in the Twilight Town Mansion. Sora and his friends venture into the unknown area. While entering, Nobodies attack them; while being attacked Axel joins Sora in battle. In order to defeat the Nobodies, Axel puts all of his life force into an ultimate attack and dies, but not before opening another rift to The World That Never Was.
It can also be speculated that the area of final battle with Xemnas is in the Realm of In Between, or the Realm of Nothingness.
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.