The Partner

 The Partner is the alias used by The Doctor's arch nemesis, The Master. He disguised himself as a man who had been manipulating The Tenth Doctor's life by somehow orchestrating dangerous situations that he was thrust into.

The Doctor first learned of The Partner during the episode Kandy Land, where the entire planet had been turned into a massive piece of candy by the Kandy Man. It is discovered that the Kandy Man was rebuilt by The Partner as a sort of experiment to see how The Doctor could handle a catastrophe at that scale.

He reveals himself as The Master after The Doctor, and Gwen free him from his prison which is revealed to be his TARDIS. (Rebirth Story: Four By Four) He was imprisoned inside it after it had went through the Wormhole Generator used by The Doctor during the destruction of The Time Well. (Rebirth Story: The Time Well Part Two)

Appearance
The Partner was an older looking man with short black hair that was combed over. He had a somewhat gaunt apperance in his face. He also seemed to be wearing an assortment of purple clothing which consisted of a purple bucaneer's coat, purple pants, a purple kravat, and brown leather boots.