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.

Sora and Riku briefly entered the Realm of Nothingness during their final battle with Xemnas (i.e. in his final form).

DiZ
After being banished from Hollow Bastion by Xehanort, and his apprentices, Ansem the Wise wandered into the Realm of Nothingness and became DiZ. DiZ only managed to stay alive and prevent himself from going insane by continuing to think and write.

Keyblade
It is unknown if this realm has chosen a Keyblade master or not. If so it is most likely Kairi, as she is the only known, human, fourth wielder of the Keyblade, and received her Keyblade from Riku shortly after remembering her past. This could be possible, since it is another one of the realms and each realm has a Keyblade master.

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.