The Master's Defender (マスターキーパー Masutā Kīpā?) is Master Eraqus's Keyblade, later passed on to Master Aqua, and appears in Kingdom Hearts Birth by Sleep and Kingdom Hearts Birth by Sleep Final Mix. As a digital simulation of Master Eraqus, the Armor of the Master also wields this blade.
Kingdom Hearts Union Cross
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 she finds Master Eraqus's Keyblade.
Following Master 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.
A Fragmentary Passage
Aqua continues to wield the keyblade throughout her journey through the Realm of Darkness. But upon reaching the dark side of Destiny Islands in attempt to give Mickey a chance to close the Door to Darkness along with Sora and Riku. Aqua used the blade to beat the heartless back and after the mission had been successful she watches as the islands and worlds return to as they were before; the Master's Defender remains on the island while Aqua herself returns to the Dark Realm.
Kingdom Hearts III
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, Master's Defender's grip is squared. This, in conjunction with the fact that the Keyblade's teeth are in the shape of an "E", suggests that the design is a reference to Square Enix: as well as "Eraqus", its initial wielder, being an anagram of "Square". 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's Defender'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.