Jeffrey "Jeff" the spider is an extremely generous spider who supposedly is the son of Billy, since Billy is the one who hatched him from his egg. Since Billy hates bugs, especially spiders, he repeatedly tries to kill Jeff, despite his kindness. Jeff's best friend is Fred Fredburger and both of them now work, along with Hoss, for a superhero team called "Underfist."
In FusionFall he appeared for the first time in the Birthday Bash event, and his model, like Zon's and Ship's, didn't move. He later appeared at Habitat Homes during the Mother's Day event looking for the Blowfish egg after Billy kicked it, though he is now gone. Like Zon and Ship, Jeff received animation when added once more after the Birthday Bash.
Jeff appeared during the 2019 Halloween event. He could be found outside the graveyard at Eternal Vistas. He and Fred Fredburger came from the Underworld to go trick-or-treating. Unfortunately, Pumpkin Spawns followed them and infested the graveyard. Jeff gave out the mission, Hallows Eve Helper, where he asked the player to defeat 3000 Pumpkin Spawns and rewarded them with the Jack 'O Lantern Head if they completed their task. Like Fred Fredburger, Jeff was given voice clips ripped directly from The Grim Adventures of Billy and Mandy.
You can get a Jeff Helmet, in addition to his eggs as thrown weapons, from defeating fusion monsters.