"Lumpy" was a corrupted Dalek, left in a state of goodness after being interrogated by two Cybermen. When being rescued by the Twelfth Doctor, he tagged along for a mission throughout Telos, Sontar and the Starbane. It later turned out that he hadn't actually been turned "good", but was instead reprogrammed to be so the Doctor would help him find the pieces of the Orb of Fates and then take him to the Dalek-infested Starbane. However, during previous reprogramming sessions, the Doctor had found a way to access Lumpy's controls remotely and used this ability once Lumpy had returned to the Starbane and the other Daleks had returned him to his natural Dalek personality, so that the Doctor could prevent the Daleks from gaining control of the Orb. (GAME: The Doctor and the Dalek)
Behind the scenes
Lumpy shares many similarities with Rusty from Into the Dalek. Both were named by the Twelfth Doctor and both claimed to have turned against their race after their nature was altered. Both Daleks appeared in stories written by Phil Ford.