The Six

The Six was the main alias used by the sixth incarnation of a Renegade Time Lord who suffered from regenerative dissonance.

Post-mortem
When Liv Chenka called the Nine a psychopath, he corrected her, saying that he was a kleptomaniac, and the psychopath was "three regenerations back".

Angered by River Song's plan of escaping, the Nine considered letting the Six "off the leash". The Six, in turn, found the Nine's torture of River, Liv, Helen, Charley and Bliss to be "fun". (AUDIO: Companion Piece)

When trying to guess the identity of, Liv believed him to be the Six. (AUDIO: Day of the Master)

When the Twelve tapped into her old personalities, the Six threatened her and the Eighth Doctor. The Twelve made her sixth incarnation apologise, and described him as a "horrible little man". (AUDIO: Planet of the Ogrons)

Personality
The Six was extremely violent and also very unstable. He would often try to get his future incarnations to butcher their way through a situation, regardless of whether the victims could be of use. (AUDIO: The Eleven, Companion Piece) The Eighth Doctor described him as a psychopath, (AUDIO: Planet of the Ogrons) as did the Nine himself. (AUDIO: Companion Piece)