|“||This planet will pay in blood. They were our family.||„|
|~ Kist revealing his plans to avenge his dead family by destroying Earth.|
Kist Magg Thek Lutiven-Day Slitheen (also known as Tim Jeffrey) is a member of the Slitheen Family, Korst Gogg Thek Lutiven-Day Slitheen's father, Glune Fex Fize Sharlaveer-Slam Slitheen and Florm Rox Fey Fenerill-Slam Slitheen's brother and the secondary antagonist in The Sarah Jane Adventures episode Revenge of the Slitheen, and the posthumous antagonist in the episode Lost Boy.
In his human disguise of Tim Jeffery, he is portrayed by Ian Midlane, who also voices his true form which is portrayed by Paul Kasey.
Revenge of the Slitheen
In 2008, Kist along his brother, son and several other Slitheen came to Earth in order to put transducers on it which would absorb the Sun's energy and destroy the planet in order to avenge the Passamer-Day branch of their family (who died in the Doctor Who episode World War Three). Whilst on Earth, Kist, Glune and Korst went undercover in Park Vale School where Kist killed and used the body of a science teacher named Tim Jeffery, Glune used the body of the Head Teacher Greg Blakeman and Korst used the body of a student named Carl.
The machine the Slitheen where using for their plan was malfunctioning but Kist and Korst as Jeffery and Carl were able to get equations to power it from a student at the school named Luke Smith (who was unaware of their plan). Later Kist discovered Luke's friend Maria Jackson was in the IT room and removed his human disguise and pursues her and her classmate Clyde. Korst pretends to led to the two to safety before revealing himself to be a Slitheen to them too but the two along with Luke are able to escape the two Slitheen with help from Luke's adoptive mother Sarah Jane. Later in their control room, Kist, Glune and Korst are able to get the machine working and Korst pulls the lever and turns off the Sun. After discovering that Sarah Jane and the kids are back, Glune goes to kill them whilst Kist and Korst stay behind in the room.
Shortly afterwards, Kist and Korst feel pain and realise that something is wrong with one of their family. Korst alerts Kist to what is on the control room's computer screen, which shows the remains of Glune, who had exploded after Maria threw vinegar (which contains Acetic acide, which is lethal to Raxacoricofallapatorians) on him. Realizing that they are under attack, Kist calls for the other Slitheen to come to the school whilst reassuring a worried Korst.
Soon after, Sarah Jane and the others appear and Luke tricks the Slitheen into thinking that the machine won't take the Sun's energy and turns it off before using Sarah Jane's sonic lipstick to make the machine malfunction. The control room starts to explode and most of the Slitheen flee whilst Kist and Korst get trapped in the door of the room whilst trying to follow Sarah Jane and the others out. Kist begs Sarah Jane to at least save his son. Before Sarah Jane can do anything, the room explodes causing Kist to disintegrate and Korst disappears in a green light causing the gang too think that both Slitheen are dead, much to Sarah Jane's sadness as she believes that she allowed a child to die.
The Lost Boy
Kist is mentioned three times in the episode by Korst (who had managed to teleport to safety) and appears in a flashback.