|Jeff the Spider|
|Relatives|| Billy ("father")|
Spider hatchlings (children)
Velma Green (ex-fiance)
|Real world information|
|Appears in|| The Grim Adventures of Billy & Mandy|
Underfist: Halloween Bash
|First appearance||"The Crawling Niceness"|
|Voice actor||Maxwell Atoms|
For reasons unknown, and that were never touched upon in the series, the spider egg that contained the spider that would become known as "Jeff" was stored inside Grim's magical trunk. Given that the natures of the items that Grim usually stores in his trunk are items that are cursed or dangerous, Jeff's species is probably dangerous. Depite only being shown in U'''''nderfist: Halloween Bash, Jeff is close friends with Fred Fredburger.'
Jeff the spider made his first appearance in The Crawling Niceness; Billy, despite having been told not to, went rummaging through Grim's trunk and found an egg. The ever-dense and deluded Billy, thinking he would get a giant duck if he were to hatch it, went forward with 'incubating' it, and then it just seemed to crushed by Billy sitting it. In reality, the egg had hatched, and the baby spider within, (at that time still tiny) ended up on Billy's back, hearing Billy say that if something had been in the egg, he would have named it "Jeffery". 'Jeffery', now, (and forever) thinking that Billy is his parent, introduces himself, and sends Billy screaming out of the basement. At times it is difficult to conclude whether Jeff is a male spider or a female spider, as he laid thousands of eggs in his web, but he still got married to Velma Green. He has also used the same love to Mandy. It is later revealed in "The Anger of Velma Queen" that Billy never lets him see his mom, probably referring to Mandy due to the fact that she is always around Billy and when she gives him advice on how to act around Billy.
Jeff is described by Mandy as being both "too nice" and "basically insane".