Derek (Survival)

Derek was a human who in 1989 was transported to the Cheetah World by a Kitling.

Biography
Derek found Midge and Shreela, two other humans who were being hunted by the Cheetah People. Ace helped them try to catch a Cheetah Person but she failed. Instead they caught the Seventh Doctor. When Ace got infected with the Cheetah Virus she helped them get home. Derek was grateful and said, "Thanks Doctor, Thanks Ace," unlike Sergeant Paterson, who just walked away. (TV:  Survival)

Behind the scenes

 * Although he survived in the televison story, in the novel he was killed by Midge after he got off the Cheetah World.