|“||When you are older, You'll learn not to make snap decisions||„|
|~ Dax to Korst when he tries to kill Sarah Jane Smith|
Dax Fex Fize Slitheen is the husband of Bloorm, a member of the Slitheen Family and one of the antagonists in The Sarah Jane Adventures episode The Lost Boy. He and his wife and their commander Korst Gogg Thek Lutiven-Day Slitheen allied with an Xylok who told them, it would help them get revenge on Sarah Jane Smith. Dax and Bloorm disguised themselves as Jay and Heidi Stafford whilst Korst disguised himself as a child prodigy named Nathan Goss. "Jay" and "Heidi" claimed that Sarah Jane's adopted son Luke was in fact their long lost son Ashley and were given custody of him, but kept him at their home and wouldn't let him out to be with his friends and contacted the Xylok via their TV saying they "have the boy".
Sometime later, Korst (disguised as Nathan) comes to them before catching Luke trying to escape and they reveal to him that they are Slitheen, but Luke is confused to why they used skinny disguises as Slitheen normally use the bodies of fat people and they reveal that they have improved their technology and are able to skinny peoples bodies now. Korst reveals himself to Luke as they had encountered each other before in Revenge of the Slitheen and plans to get revenge for his family's deaths. The three Slitheen then take Luke to the Pharos Institute and start to harvest his telekinetic powers but he is able to break free and is pursued by Korst, but is able to escape.
Later Sarah Jane, her friend Maria Jackson and her father Alan discover that Sarah Jane's computer Mr. Smith is in fact the Xylok and go to the Pharos Institute and confront Dax and Bloorm with bottles of vinegar (which can kill Raxacoricofallapatorians) but then Korst comes back and holds Maria hostage, but let's her go when Sarah Jane and Alan dispose of the vinegar. The Slitheen then reveal that they are in league with Mr. Smith as he said he would help them get revenge for their family's deaths but are confused to why Sarah Jane is there and discover that Mr. Smith knew Luke would escape and plans to use him to free his fellow Xylok by crashing the Moon into Earth. Dax along with Bloorm and Korst form a truce with Sarah Jane, Maria and Alan and using Korst's teleporter, Sarah Jane arrives in her attic and confronts Mr. Smith. She then calls for her robot dog K9 who battles Mr. Smith and she put a virus into the computer which makes him forget his plan and gives him a new purpose: to safeguard Earth. After this Dax along with Bloorm and Korst leave Earth in peace.
As Jay, he is played by Jay Simpson who also voices his Slitheen form which is played by Paul Kasey.