Former Lord President of Gallifrey.
The Timekeeper started off as the keeper of the Matrix. Events led to The Timekeeper accessing records within the Matrix, and he was horrified by what he saw. He eventually named himself a renegade, choosing the name The Timekeeper. Some time after he left Gallifrey, he attempted to form an alliance with a scientist named Davros. This alliance fell through and eventually led to the Dalek Timewar. He hunted Ashilon for a time, presumably to get Ashilon’s book from her. He eventually killed the President Borusa and took the office of Lord President of Gallifrey in a successful coup, aided by the Sontarans.
His true motives, as well as the reasoning behind his choice of name, have yet to be discovered. The only clue that Ashilon and her crew have is that he wanted Ashilon’s book, as he believed it would help him avert some catastrophe that Rassilon had set into motion long ago.
The Timekeeper is currently dead, killed during the Fall of Gallifrey by a combination of Dalek fire and a grenade from Old Man.