Master's Defender

The Master Keeper (マスターキーパー) is Master Eraqus's Keyblade, later passed on to Master Aqua, and appears in Kingdom Hearts Birth by Sleep. As a digital simulation of Master Eraqus, the Armor of the Master also wields this blade.

Like his erstwhile friend Master Xehanort, Master Eraqus rarely draws his Keyblade. When he does fight, he is able to use the Keyblade to channel his own light magic. However, even after Master Eraqus is slain by Xehanort's hand, and the Land of Departure is devoured by darkness, his Keyblade remains. After destroying Vanitas, Aqua takes a comatose Ventus back to the Land of Departure, where they find Eraqus's Keyblade. Following Eraqus's prior instructions, Aqua uses the Keyblade to transform the world into Castle Oblivion, a labyrinth which can only be fully navigated by herself, and hides Ventus's body within. Although Aqua keeps this Keyblade, she does not actually wield it until after battling Terra-Xehanort with her own Keyblade and sacrificing it to save the remnants of Terra within him. She continues to wield it throughout her journey through the Realm of Darkness.

Design
The Master Keeper has a smooth, cylindrical, grey shaft that tapers outward at both ends. It has a boxy guard, made up of two small, light grey boxes and copper rod. The base of the guard connects to the grip, and both sport several small indentations lining them. The grip itself is somewhat unique in that, while most Keyblade grips are cylindrical, the Master Keeper's grip is squared. The teeth are in the shape of an "E". The "E"-shaped teeth and the square grip are likely an allusion to "Square-Enix", which Eraqus's name also references. Its Keychain is made of several simple chain links and its token is the Mark of Mastery symbol that appears on Eraqus's, Ventus's, Terra's, and Aqua's clothing.

The Master Keeper's name reflects that it is used exclusively by characters with the title of "Master". "Keeper" can mean "guardian", which may reference its use by Keyblade Masters to guard the Land of Departure.

Kingdom Hearts III
In the teaser trailer for Kingdom Hearts III, Sora picks up the Master Keeper off Destiny Islands's beach. It is currently unknown whether or not the Master Keeper will appear in the actual game, although the game's premise of characters searching for previous Keyblade wielders implies that it will.