May my heart be my guiding key.
Luxu is a Keyblade Master and an apprentice of the Master of Masters. He was given the role to watch over events with his Keyblade in order to allow his Master to write the Book of Prophecies, as well as to hide a mysterious Black Box.
After passing his Keyblade down to one of the five new Union leaders, Luxu cast aside his original body, allowing his heart to sojourn itself in vessel after vessel for centuries, all while keeping a close eye on his Keyblade from a distance. When a prophesied "dark seeker" arose to begin another Keyblade War, Luxu stepped in and donned the mask of his ally in order to finish his role and eventually reclaim his Keyblade when the time was right.
Luxu is a boy wearing a black coat.
His name is derived from the Latin word "luxuria", or Lust, which is symbolized by a scorpion in the Ancrene Wisse.
Luxu was one of six apprentices to the enigmatic Master of Masters, the writer of the Book of Prophecies, a tome that contained knowledge of the events to come. One day, Luxu was summoned by his Master to the Foretellers' meeting chamber, where the Master bequeathed to him the Gazing Eye Keyblade, at the time he referred to as "No Name". The Master reveals to Luxu that the eye within the Keyblade was in fact his, telling his apprentice that his role was to pass down the Keyblade to his future apprentice and so forth. This prompts Luxu to realize that the Book of Prophecies was a result of future foresight from the process of passing the Keyblade and the eye within it over the ages, to which the Master congratulates the bewildered boy for accomplishing.
Continuing, the Master explains that Luxu wouldn't receive a copy of the Book of Prophecies in order to prevent any temporal paradoxes from occurring and that he was alone from here on out. Luxu questions if he would really have to go alone, asking of the other Foretellers, though the Master assures that it was "minor details". The Master then reveals a mysterious Black Box, telling Luxu to stay out of sight with it, all while observing over the others with his Keyblade and his own eyes. Luxu asks what was in the box, though the Master tells him it was a secret, and that he mustn't even open it. This causes Luxu to become even more curious, so the Master decides to indulge the boy once he promised not to tell anyone nor ever open it. Once Luxu agreed, the Master whispers its contents to him. Shocked upon hearing what was in it, Luxu asks why, though the Master tells him he will see.
With his path set ahead of him, Luxu trudged through a barren desert with both his Keyblade and the Black Box in tow. Observing the weapon, he recites his Master's mantra: "May my heart be my guiding key". He disappeared shortly before the Master of Masters did.
Luxu is mentioned only a few times by Chirithy and the Player's Foreteller. Gula informs Skuld and the Player that Luxu is the only one who might be able to find the Master, and that Ava is searching for him. Ava tracks down Luxu to the outskirts of Daybreak Town. Luxu explains his mission, and claims that it is impossible to avoid the Keyblade War, and that he will set off after this world ends. Ava accuses him of being the traitor responsible for sowing distrust between the unions, and he brandishes his Keyblade.
Luxu proceeds to tell Ava the truth in secret while claiming the Keyblade War was all but inevitable; any attempt to stop it was futile. When told by Luxu that the Master favored the events to come as opposed to the greater good of the world, Ava accuses Luxu to be taking advantage of the Master's intentions for himself. Drawing her Keyblade, Ava attacks Luxu, the resulting clash of their Keyblades sends a shockwave across Daybreak Town, signifying the start of the Keyblade War.
As the Keyblade War reached its conclusion, Luxu watched in silence from afar with the Black Box at his side as he used the Gazing Eye to relay the events to the Master of Masters.
Having seen the Keyblade War unfold, Luxu continued to carry out the role the Master of Masters had given him. He eventually bequeathed the Gazing Eye to one of the five new Union leaders, and watched as they were sent into another worldline. This however, left him without a Keyblade of his own, and in order to fulfil his final task, Luxu had to cast his own body aside and use different vessels in order to survive.
After many lives, watching as his Keyblade was successfully passed down from master to apprentice, Luxu decided to intervene as the Gazing Eye was bequeathed to Xehanort, a Keyblade Master fascinated with both the darkness and the Keyblade War. With a new body and identity - Braig - he assumed the role of a fool who desired the power of the Keyblade in order to get close to Xehanort.
Luxu is responsible for writing the eleventh, twelfth, and thirteenth Secret Reports, detailing his mission after witnessing the Keyblade War and the countless lives he had lived over the years before obtaining his final vessel as the ally of the scapegoat who would re-enact another Keyblade War, an act that would allow the other Foretellers and the Master of Masters to return.
Though not much is known about his level of strength, it is implied to be on par with that of the other Foretellers. He is shown to be able to at least fight with a Keyblade and has some strength as well, being able to move heavy objects like the box.
Not long after bequeathing his Keyblade, Luxu utilized an unknown method to transfer his heart to vessel after vessel even without a Keyblade, using this method to prolong his life for hundreds of years in order to fulfill his role.
Luxu is given a Keyblade known as the Gazing Eye by the Master of Masters. The weapon has a spiky, black guard resembling demonic wings, a horned, demonic head just above the handle, two barbed shafts, and teeth resembling a battle axe split into three almost claw-like protrusions. Its shaft and head are adorned with several twisting, silver patterns. It also contains two demonic blue eyes, with one on the Keychain and the other one on the head, which in fact is his Master's "gazing eye".