Gustave Lytton

"I don't think I ever misjudged anybody quite as badly as I did Lytton."

- The Doctor

Commander Lytton was a mercenary who at different times, worked for the Daleks and later for the Cryons.

History
Lytton was born on the satellite Riften 5, orbiting the planet Vita 15. He knew of the Time Lords. (DW: Attack of the Cybermen)

When the Doctor encountered Lytton he was leading a squad of Dalek Troopers in a plot to rescue Davros from imprisonment in stasis on a human prison space station. ) When Davros altered some of the Daleks to be loyal to him and attempted to seize control from the Dalek Supreme, Lytton was one of the few survivors of the ensuing battle and was left trapped in London in the year 1984. (DW: Resurrection of the Daleks

Consequently, Cryon resistance fighters contacted him. With Cryon planet, Telos, occupied by Cybermen, they sought help from him in freeing themselves. Lytton assembled underworld contacts in appeared a plot order to rob a diamond merchant. He actually planned on contacting the Cybermen living in the London sewers in order to win the trust of the Cybermen, so that he could betray them on behalf of the Cryons.

Lytton met the Doctor, who recognized him from the previous encounter in the space station and distrusted him immediately, even after the Cybermen took Lytton prisoner as well. On Telos, where the Cybermen transported Lytton, the Doctor and the Doctor's companion, Peri it was revealed to the Doctor that Lytton was working for the Cryons and not for the Cybermen themselves.

Once Lytton's treachery to the Cybermen was conclusively proved to the Cybermen, the Cyber-Controller ordered that Lytton undergo the cyber-conversion process. The Doctor tried to free Lytton from his fate as a Cyberman, a partially converted Lytton died fighting the Cyber-Controller. Shortly after, the Doctor admitted that he had badly misjudged Lytton and his intentions. (DW: Attack of the Cybermen)