Rossiter

Rossiter was a member of the Vinvocci, an alien race. Along with his partner, Addams, Rossiter infiltrated the staff of billionaire Joshua Naismith on Earth in order to salvage a piece of equipment referred to by Naismith as the Immortality Gate, and which the tycoon had recovered following the fall of Torchwood. Using a' Shimmer' to disguise himself as human, he (along with the similarly disguised Addams) infiltrated Naismith's inner circle. Their efforts to spirit the Gate away were disrupted by the arrival of the Doctor and Wilfred Mott. Rossiter and Addams were reluctantly recruited by the Doctor to stop the Master's scheme to take over the Earth and facilitate the return of the Time Lords. He helped the Doctor escape the Master's clutches and then using Addams' ship he, along with Wilf, destroyed several missiles aimed at them by the Master. When Addams dropped off the Doctor and Wilf at Naismith's mansion for a final showdown with the Master, they presumably departed Earth. (DW: The End of Time)

Rossiter was a panicky, skittish individual; After knocking out the Master, he surprisedly declared that he had never hit anyone before, and when he and Wilf were manning the Hesperus' Asteroid lasers against The Master Race's missile attack, he screamed and whelped throughout the attack.

Trivia

 * Rossiter is apparently one inch taller than the Master.