Well, I could lie and tell you what I am, when I'm actually a brilliant artist, or even a scholar. I could tell you that I dream of world peace. When I'm actually planning it's destruction. The truth is what you see with your eyes, not what you hear.Master of Masters to Master Xehanort while in his younger days.
The true personality of the Master is unknown due to his contradicting manner in all his appearances. He constantly behaves in an unpredictable and eccentric manner despite his extraordinary intelligence and wisdom but his ability to stage the Keyblade War while ensuring the worlds would still exist, manipulating his own loyal apprentices with no apparent qualms despite their absolute loyalty and his apparent affection for them, indicates a machiavellian personality. (He ensured the Foretellers would begin distrusting each other by giving a Gula a wild goose chase, ensuring his rules were too restricting to make Aced act more proactive, made Ira leader due to his stubborness restricting the Foretellers to his rules, make Invi seem like a spy by telling her to ensure everyone gets along, and have Ava found the Dandelions to make her look suspicious for gathering Keyblade wielders from Unions).
His true allegiance remains unknown as although he apparently cared enough for the existence of the worlds to ensure they would survive despite this not stopping him from carrying out his mysterious plans, he was willing to let several disastrous events unfold while merely watching. In addition, he is quite capable of being serious, as displayed by his ability to strike clear worry into his extremely powerful apprentices when they asked him regarding their roles, despite often returning to his old self.
From what has been revealed of his true feelings, he is tired and bitter over having seen the fight with light and darkness go on in an endless cycle, noting that he wants to "take a break" from it and his entire plan of choosing new Union Leaders and ending the current world was all to finally put a stop to the Keyblade War. Additionally, the Master also proved to be supportive of Xehanort's view that the current state of the world has the weak steal power from the strong and unjustly claim it was righteous. All of this suggests a cynical outlook on the state of humanity and the worlds.
The Master of Masters appears as a person completely concealed in a black coat. Nothing else is known about the Master's appearance beneath the hood, although according to Chirithy he has an eye that gazes into the future. As the Master has installed one of his eyes inside the Gazing Eye, he has seemingly lost one of his eyes, although it remains unknown if he can't just regenerate or replace it.
Before Kingdom Hearts χ
When the Master of Masters was just a boy, the Keyblade War had already began between light and darkness. Though confirmed by himself that there were other Keyblade wielders at that time, he does not directly mention if they were allies or enemies, but states that the darkness took the form similar to, but not quite, that of a human. In addition, the Master of Masters also states that he didn't have a Master of his own despite being a Keyblade Master, most likely meaning he created the title himself.
Years later, the Master of Masters, having studied the χ-blade, used the skills he learned from it to take in six apprentices, Luxu, Ira, Ava, Invi, Aced and Gula, and began to train them into becoming Keyblade Masters of their own right, giving them Keyblades crafted from their own hearts and their names. The Master of Masters also created the Book of Prophecies with his Gazing Eye to view any events in the future it witnesses and copied it multiple times so all his apprentices except Luxu could have their own Book of Prophecies. Having grew disillusioned with the unending Keyblade War, he decides to end it for once by manipulating his apprentices and disappear. He first bestows Luxu the Gazing Eye and instructs him to pass it on to others so that the Gazing Eye can relay future events to him and he can write the Book of Prophecies successfully. He then orders Luxu to watch and if necessary interfere as things happen between his fellow apprentices and also gives him the Black Box, which he decides to tell Luxu about at the condition he swear not to tell anyone else, with the contents being indicated to be a copy of the Book of Prophecies despite him not giving Luxu one to prevent him from causing paradoxes but whatever it was caused Luxu great surprise, which the Master simply said that he'll see sinisterly.
After Luxu leaves, the Master of Masters then proceeds to reveal that he will disappear and makes Ira leader of the Foretellers to keep his other apprentices in check, but rebukes Ira's desire to prevent the final battle of the Keyblade War he had planned, stating it was inevitable. He then makes Aced Ira's right-hand man, with the task of taking charge should Ira prove to slack in his task, accounting in Aced's react personality to push Ira into action but considering his true intentions, and Invi the moderator, tasked with keeping watch over the others and reporting to Ira if the others tried to disobey his teachings, with the likely intention to delay the Keyblade War until Ava's task, to gather all the exceptional keyblade wielders he chose from every union to form the Dandelions and train them to venture out of the destroyed world to ensure the survival of the light and recreation of the world. Gula's task to stop the traitor in the Lost Page was clearly a set-up mission to make Gula gather the Lux needed to summon Kingdom Hearts, knowing the others will have to counter him by gathering as many Lux as possible. Finally, he creates the Chirithy to aid Keyblade wielders, with a warning to his apprentices that if the wielder falls to darkness, their Chirithy will be corrupted and become a Nightmare. At some point, he talks with Luxu and reveals his origins and confirms the existence of other Keyblade wielders, although he states he is the only one relevant, and that by sending the Dandelions away to the data world, they will eventually be able to recreate the world. He then vanishes, leaving so little traces that the Foretellers, despite their search, are unable to find the Master.
After helping Chirithy fend off the Minute Bombs in Daybreak Town, the Player goes to sleep. They have a dream in which the five Foretellers converse with their master before the Player has a chance to approach the Foretellers and their mysterious visitor, they are engulfed in darkness and wake up.
The Master of Masters is mentioned by the Secret Reports written by Luxu. It was stated that out of the five new Union leaders, there is one he didn't chose. Luxu also lamented that without the Master, the darkness began to spread even without any Keyblade War. It has been stated that the Master will return after the Keyblade War comes to a close and the Foretellers are reunited with Luxu.
The Master of Masters is seen in the secret movie Yozora. As he stands atop a building, he uses his hands to make an outline of Kingdom Hearts with the distant moon.
Master Xehanort told Xigbar and Saïx that about seventy-five years ago, he had met the Master of Masters at the Keyblade Graveyard, and how the latter would have given him his Black Coat with which to repel Darkness while browsing the different Worlds. Xehanort tells the Master that all the "Corruption" he had seen through the many Worlds he had visited had made him lose his faith in Humanity. Determined to put Order into this Chaos, he decides that it would be necessary to "remake the Worlds". He says his decision will become clear after his next Mark of Mastery Exam, but the Master of Masters tells Xehanort that the results don't matter if one has already made his decision. He says that soon Xehanort will no longer need his Black Coat, as it will be powerful enough to control the Darkness without it. Xehanort asks if he is a fortune teller, but Master of Masters will only make numerous claims about his identity before telling Xehanort his name, proving to be one of the "Lost Masters".
Currently, nothing is known about the Master's might but he is certainly an extraordinarily skilled and powerful Keyblade Master, having personally trained the incredibly strong and skilled Foretellers and Luxu, molding them into powerful Keyblade Masters themselves, while still being superior to their level of might, deemed by Gula as having the power to forcibly stop all of them from causing the Keyblade War. All the Foretellers have shown absolute respect of his strength and even fear of his power, able to effortlessly intimidate even the arrogant and powerful Aced merely pretending to find his disapproval of Ira becoming the leader undesirable. He is at least strong enough to casually move the heavy Black Box from a kneeling position even without aid from Luxu.
Being perhaps the oldest character alive in the series, the Master has extraordinary knowledge and in addition to foretelling and writing the future in the Book of Prophecies, he also proved able to write detailed notes about many topics such as the Keyblade and Heartless. From his study of the χ-blade, the Master has the ability to create Keyblades from the hearts of others and can also seemingly construct Keyblades of his own without needing to use the hearts of others, as he was able to create the Gazing Eye, which was not his main Keyblade, through unknown means. The Master appears to be an extremely skilled sorcerer, having been able to enchant the Gazing Eye within the No Name Keyblade he gave Luxu so from it he could perceive anything the Eye saw from anywhere throughout time, which made the Book of Prophecies he authored possible to exist, and he also seemingly enhanced the Book with the power to summon powers from the future. The trap set in the flow of time to prevent any attempts to rewrite the past and trapping those who tried inside a data simulation of their world created by him may also be another piece of the Master's magic. The Master also appears to be highly proficient in manipulating light and darkness, being capable of creating the Spirit Chirithys for the Foretellers and their Union members, creatures capable of wielding powers of light and also becoming corrupted and controlling creatures of darkness, and also creating bangles capable of siphoning and harnessing dark energy. When conversing with Luxu, the Master dimmed the light of the lamp and then lit it back up by just pointing and swinging his hand at the lamp. The Master appears to be capable of sensing whenever Kingdom Hearts is summoned, as his apprentices believe that summoning Kingdom Hearts would cause his return. Through unknown means, he was able to remain alive through the centuries after the Keyblade War with seemingly no deterioration to his vitality and power, indicating he is either immortal or used the same heart-extraction technique Luxu used to remove his heart from his body and then possess another body.
An extremely intelligent and wise man, the Master of Masters is an extraordinary strategist and manipulator, as he gave each of the Foretellers their own roles specifically based on his knowledge of how they would react to successfully ensure the Keyblade War would occur and despite the overwhelming darkness unleashed from the war, he was still able to successfully keep the light from fully expiring and prevent the darkness from completely consuming everything, allowing the worlds to exist, but separated, by having the members of the Dandelions transported to a different worldline so as to protect them and their light.
Along with that, he also successfully instructed Luxu in how to enact another Keyblade War that was so magnificently done that Luxu successfully eluded all detection in his part, to the point that even the brilliant Master Xehanort was completely fooled into believing Luxu, as Xigbar, was merely desiring the power of the Keyblade enough to join him.
Although the main Keyblade of the Master of Masters is unknown, he created and briefly wielded The Gazing Eye, a Keyblade forged from one of his eyes, which he used to see future events in order to write the Book of Prophecies.