Krislok

Krislok was a Graske who enountered Sarah Jane Smith and her friends. He was forced to act as an accomplice of the Trickster.

Biography
The Trickster rescued Krislok from his spacecraft, which had begun to burn up, in return for which Krislok worked for Trickster as a slave. (SJA: The Temptation of Sarah Jane Smith) Under the Trickster's orders, he first attempted to erase Maria Jackson from time by kidnapping her and stopping her from ever existing, as she had been protected from the Trickster by the Verron soothsayer's puzzle box. The process went slightly wrong and she briefly ended up in 1964 before being recaptured. He was later sent after her father, Alan Jackson, although Alan managed to tie him up and forced him to reveal how to bring Maria 'back in time' and rescue her from limbo. Krislok later managed to free himself and teleported away. (SJA: Whatever Happened to Sarah Jane?)

While still working for the Trickster, Krislok posed as "Oscar" in order to lure Sarah Jane to 1951. He lured her through a time fissure, to a time just prior to her parents' death, in order that she would try to change it. After the Trickster was successful in creating an alternate timeline where he manifested corporeally on Earth in 1951, the present day was a desolate wasteland where Krislok enslaved the few surviving Humans to mine for resources so he could extend his reach to other planets. Krislok re-opened the time fissure to allow Rani to tell Sarah Jane where the Trickster was set to manifest. Clyde remained behind, guarded by Krislok. After the timeline was restored and the Trickster defeated, Clyde gave Krislok the box to protect him should the Trickster ever return and seek to punish him. (SJA: The Temptation of Sarah Jane Smith)

He was, presumably, protected, as he did not return with the Trickster when the latter arrived at Sarah Jane's wedding. (SJA: The Wedding of Sarah Jane Smith)

Behind the scenes

 * Whatever Happened to Sarah Jane? does not give Krislok's name. This occurred in the second episode of The Temptation of Sarah Jane Smith. Neither episode gives his name in the credits and simply list him as "the Graske".
 * A graske is going to appear in Series 4 but its unknown if it'll be Krislok.