The Master of Masters

The "Master of Masters" is a Keyblade Master that appears Kingdom Hearts χ. He is responsible for training the five Foretellers and gifting them with the Book of Prophecies.

Before Kingdom Hearts χ
According to the 's Chirithy, the five Foretellers and one other Keyblade wielder were the apprentices of his creator, the "Master of Masters." Shortly before he disappeared, the Master of Masters assigned his disciples new names and gifted five of them with a tome from the Book of Prophecies, which taught them of the event that would doom the world.

Kingdom Hearts χ
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.

Later, when the Player has another dream of Ephemer entering the clock tower through the sewers, the Master of Masters can be seen watching him from the shadows.

Appearance
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.