Master's Defender

The Master's Defender is Master Eraqus's Keyblade. It is later passed on to Master Aqua, and it appears in Kingdom Hearts Birth by Sleep and Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-. A digital simulation of the Keyblade is wielded by Armor of Eraqus, while an illusory copy is wielded by Phantom Aqua.

Kingdom Hearts Birth by Sleep
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.

Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-
Aqua continues to wield this Keyblade as she advances through the Realm of Darkness. During her journey, she encounters a phantom of herself, who wields an illusory copy of the Master's Defender.

Design
The Master's Defender 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's Defender'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 Terra's Mark symbol that appears on Eraqus's, Ventus's, Terra's, and Aqua's clothing.

The Master's Defender's Japanese name reflects that it is used exclusively by characters with the title of "Master". "Defender" 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's Defender from Destiny Islands's beach and examines it.

Master Keeper Meister Beschützer