Jeff is actually a very lazy and crazy elf, who does not want to work in Santa Claus's workshop. He was sent to be Timmy's replacement fairy while his godparents were getting their Godparenting license renewed. He can only create toys.
He wears a green Christmas elf tunic, with a long hat with a bell at the end and several strands of hair poking out from underneath. His eyes are colored red, and he speaks with a gruff voice. He carries a wand, but he can only wish up toys, and instead of a "POOF!" his wishes make a belching sound. He also has fairy wings on his back, although it's not known if they are even real.
In his appearance in "Abra-Catastrophe!", Jeff's hair is more visible from underneath his hat, and it is colored red.
Whenever Timmy wished for something, Jeff would either hesitate and recommend something else, or wish up a toy relating to whatever Timmy wished for. For instance, Timmy wished to go to the beach, and Jeff wished him up a pail. The fact is that Jeff can only create small toys. Eventually, Santa Claus shows up at Timmy's house to reclaim Jeff. Santa takes a protesting Jeff back to the North Pole, while the other elves (in a creepy, droning type of way) tell Jeff he is "one of them" now. He was last seen screaming as he was being brought back to the North Pole in Santa's sleigh.
Jeff later a cameo at Timmy's Fairyversary party in "Abra-Catastrophe!", although he had no lines.