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 posed as Jay and Heidi Stafford and claimed that Sarah Jane Smith adopted son Luke was infant 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 via their TV a being called Xylok saying they "have the boy". Sometime later a child prodigy named Nathan Goss 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. Nathan also reveals himself to be a Slitheen too but not just any one by Korst Gogg Thek Lutiven-Day Slitheen, the same young Slitheen Luke encountered in Revenge of the Slitheen and plans to get revenge for his family's death. 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.