Lubber, (Lubbermouth according to the end credits,) is a worm-like creature from The Underworld. She was a former classmate of Grim's from "The High School of the Dead", before she was sentenced to become guardian of 'The Bottomless Pit' where the supposed 'secret of the universe' was kept.
According to Grim, Lubber had a crush on him and wouldn't stop following him everywhere around school, and one day, when one of the other kids poked fun at Grim because of his 'shadow', Lubber turned around and ate the kid whole. Soon, Lubber had almost eaten all the other students until she was reprimanded by the school principal. Lubber was cursed to guard 'the secret of the universe,' either for an eternity or at least until the curse was somehow lifted ("which ever came first, really" according to Grim).
After studying up on her curse, Lubber learned that if 'the secret of the universe' should be seen by the eyes of a mortal, then the curse would be lifted from Lubber and the mortal in question would then be subject to the curse instead.
Desiring her freedom, and to party "with total strangers", Lubber sent out phony decoder rings inside packs of children's cereal boxes in order to lure any such kid who would be presumptuous enough to think that they could know 'the secret of the universe'. Billy, along with most of the youth of Endsville, eat their way through boxes of cereal to decode the secret message.
Billy, Mandy, and Grim headed to the hole first to discover the secret, but just as Mandy was about to find out, Mindy beat her to it. It turned out that the secret was all a trick and Mindy became the new guardian after somebody pushed her into the hole, thereby freeing Lubber from her curse.
- Her name, Lubber, may be a pun and lubber is the fat of a whale, or be a pun on the word "lover" or both.
- Lubber appears in the flashback sequences during Wrath of the Spider Queen.