Gareth Jenkins (A Fix with Sontarans)

Gareth Jenkins assisted the Doctor in defeating the Sontaran Group Marshall Nathan and his confederate.

The Doctor transported Gareth Jenkins from Earth to the TARDIS by accident, Gareth Jenkins had on a boy-sized version of the Doctor's own clothes. He also knew how to operate the TARDIS controls from watching the Doctor do so on Doctor Who and knew what switches to press on it to kill the two Sontarans. The Doctor revealed that in the year 2001, when Sontarans invaded, that the adult Gareth would lead a resistance movement which turns away the invasion fleet. (DW: A Fix with Sontarans)

Behind the Scenes
A Fix with Sontarans probably does not take place in the continuity of the Doctor Who Universe. Another, unrelated Gareth Jenkins works with Big Finish Productions.