Zim was an overseer.
He once put the mind of Hans Vogel in the Third Doctor's body into him after attempting to wipe the Doctor's mind using a mind thresher. After he thought the Doctor's consciousness had been completely wiped, he placed Vogel's mind inside of his brain. Due to the complexity of a Time Lord mind, however, Zim was unable to completely eliminate all traces of the Doctor's consciousness. This resulted in a Meta-Crisis which threatened to destroy both the Doctor and Vogel's minds. In the end, the Doctor was able to remove Vogel from his mind by sabotaging Zim's facility. (AUDIO: The Hidden Realm)