Realm of Nothingness

The Realm of Nothingness is the fourth and final realm in the Kingdom Hearts games. Nothing exists here and if one stays here too long, they will not exist either. The only way to survive here is to remember your past.

It is unknown if this realm has chosen a keyblader or not. If so it is most likely Kairi, as she is the only known fourth wielder of the keyblade, and received her keyblade from Riku shortly after remembering her past.

Others have speculated that this realm's wielder may be Roxas, however this is highly unlikely since Roxas' ability to wield the keyblade is inherited directly by being Sora's Nobody.

DiZ
After being banished from Hollow Bastion by Xehanort, Braig, Dilan, Even, and Ienzo, Ansem the Wise wandered into the Realm of Nothingness and became DiZ. DiZ only survived here by remembering his past.

Trivia

 * When Sora finally visits The World That Never Was, (which dwells within the Realm of Nothingness), he would have finally visited all 4 realms.