Carl Harris

Private Harris was a UNIT soldier during Operation Blue Sky in 2009. His call sign was Greyhound Fifteen.

Entering the ATMOS factory with other UNIT troops during their raid of the complex, Private Steve Gray and he discovered a cloning pool in the building's basement, the latter correctly identifying a creature in the pool as a human embryo that breathes the liquid it's contained in. The Sontaran General Staal commended him for his leadership skills and intelligence.

Staal hypnotised the two troopers and had them capture Martha Jones so the Sontarans could make a clone of her. (TV: The Sontaran Stratagem) While hypnotised, 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 disposed of them during the battle with UNIT at the ATMOS factory. (TV: The Poison Sky)

Behind the scenes
In TV: The Sontaran Stratagem/The Poison Sky, only Harris's last name is given; his first name, Carl, is given in The Time Traveller's Almanac.