Realm of Nothingness

Realm of Nothingness is the fourth and final realm in the Kingdom Hearts games. Nothing exists here and if one stays in there for too long, they will cease to exist. The only way to survive here is to remember your past.

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.

Keyblade
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. However this is very possible, since it is another one of the realms and each realm has a wielder.

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. However, Roxas, being a nobody, would suite the Realm of Nothingness.