The Seven

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

Post-mortem
He was used by the Eleven while posing as a scientific advisor. (AUDIO: World of Damnation, Sweet Salvation)

When the Nine was aiming to kidnap all of the Doctor's companions, River Song told him to retrieve Katarina's dead body, as that would be the only way to get a "complete collection". In fact, her body was being guarded by a younger version of River, who confronted the Time Lord, giving the older River a chance at escaping. It was the Seven who realised her plan and warned his future self to go back to the TARDIS. (AUDIO: Companion Piece)

Personality
The Seven was known for being rude and abrasive. Indeed, the Nine thought that him congratulating the Eleven was rare. He was also the more scientifically inclined incarnation. (AUDIO: World of Damnation, Sweet Salvation)