Lau'Tan

Lau'Tan was a Gizou assassin who first encountered the Tenth Doctor while posing as the Krillitane, Brother Lassar.

Biography
Using Lassar's Human alias "Lucas Finch", he served as prosecutor when the Tenth Doctor was put on trial by the Shadow Proclamation. However, it was later revealed that "Finch" was a shapeshifting Gizou in disguise, and he was arrested by the Proclamation. The Gizou was working under orders from the Doctor's Advocate. (IDW: Fugitive)

Still working with the Advocate to ensnare the Doctor, Lau'Tan then posed as an Enochian during their invasion of 2009 London. Lau'Tan admitted to having spent two years convincing the Krillitanes that he was Lassar, and less time convincing the Shadow Proclamation. He did not know whether the real Lassar survived his defeat at Deffry Vale High School, nor did he care. (IDW: Don't Step on the Grass)

The Doctor next met Lau'Tan on an unnamed planet approximately 20,000 years later and five star systems away, and about two years later from Lau'Tan's point of view. The planet was a battlefield, divided between two groups: the Soul Free and the Terror Farmers. Now in his natural form and going by his true name, Lau'Tan was in command of the Soul Free, in opposition to the Advocate who was leading the Terror Farmers. Lau'Tan joined forces with the Doctor against his former employer. (IDW: Final Sacrifice)