Carl Harris

Private Carl Harris was a UNIT soldier during Operation Blue Sky who's call sign was Greyhound 15.

Entering the ATMOS factory with other UNIT troops during their raid of the complex, he and Private Steve Gray discovered a cloning pool in the building's basement. The Sontaran General Staal commended him for his leadership skills.

Staal hypnotised the two troopers in order to have them capture Martha Jones so that the Sontarans could make a clone of her. While hypnotized, they regarded Staal as their commanding officer and considered it their duty to obey Sontaran commands.

Harris later teleported the Doctor's TARDIS onboard the Sontaran flagship. Soon after, they reported for duty to a Sontaran soldier who then disposed of them during the battle with UNIT at the ATMOS factory (DW: The Sontaran Stratagem / The Poison Sky).

Alternate Timeline
In an alternate universe he was the UNIT officer on scene when the Doctor's body was recovered from the Thames and reported that the Doctor was dead. (DW: Turn Left)

Behind the scenes
His first name, Carl, was given in The Time Traveller's Almanac.