Gramma Stuffum is a fat old lady obsessed with force-feeding her disgusting food to children to fatten them up, believing that a child that is too fat to move can't get into trouble. She speaks with a German accent.
She commands an army of living food that aggressively feeds itself to kids. Among these are Gramma Stuffum's constant companions, Liver and Onions. Stuffum has twice also exhibited above-average strength.
She first appeared in Operation: N.O.-P.O.W.U.H., in which she was interviewed by the Delightful Children From Down The Lane, who wanted to find a villain to bring havoc upon the KND. They convinced her to do so by saying their foes were famished. Gramma Stuffum then attacks Sector V Treehouse with her food army while their power is out. She is able to fatten Numbuh 1, Numbuh 3, Numbuh 4 and Numbuh 5 into a state of helplessness, but Numbuh 2, who has an enormous appetite and is more tolerant of her disgusting dishes, proves much more difficult to subdue.
When he is finally full and about to give up, the Hamsters return and eat the rest of her food. Numbuh 2 then fires Gramma Stuffum back to the Delightful Mansion From Down The Lane, where she decides to feed the Delightful Children.
In Operation: F.O.O.D.F.I.T.E., Gramma Stuffum is apparently revealed to be a cafeteria chef at Gallagher Elementary. During lunch period, the members of Sector V have an extended battle with her and her food, which is implied to be a regular occurrence during lunchtime. Gramma Stuffum then summons the monstrous Slamwich, which overwhelms the children until the super-powered hamster Joaquin appears and eats it. During the battle sequences are lyrically altered versions of the GWAR songs "The Private Pain of Techno Destructo" and "Gor-Gor". GWAR provided the music for this episode under the name "RAWG". After her defeat, she tells them to prepare for lunch the next day and Numbuh 4 throws a chicken leg at her, hitting her in the head as he tells her to get a real cookbook.
In Operation: V.I.D.E.O.G.A.M.E., she takes over the Sprinkle Puff Donut Shop and wishes to fill the donuts with spinach, liver and olives (and even haggis if the player is using Numbuh 86 as an active operative). She is the antagonist and boss of Operation: D.O.N.U.T.T.Y.
In Operation: Z.E.R.O., Numbuh 2 compliments Gramma Stuffum after eating a particularly delicious food of hers, only to be disgusted when she tells him that it was fried platypus intestine. She was last seen in Operation: I.N.T.E.R.V.I.E.W.S. where she baked a tusk pie, which was soon stolen by Mr. Wink and Mr. Fibb, then Robin Food.
She controls an army of living meals who can force feed themselves to any kid until they become full. She controls different kinds of living meals, all being made from stuff that kids won't like to eat.
Her army is finite, as an army of hamsters can easily much down all of them. Her army is also useless against kids with insane appetite, and those who eat the meals by themselves won't be affected by the meals too much.
- In Operation: F.O.O.D.F.I.T.E., she wears a witch's hat, making a slight pun with the word "slam witch" that is supposed to be a pun with "sandwich", hence the huge sandwich monster she was controlling in the episode.
- She deems even fat kids as being skinny. In her eyes, a child is fat and happy when they're too immobile to move.
- She is one of the fewest villains capable of taking down the Sector V KND with sheer ease, though the time she does it is because of the KND lacking a power source to power their weapons.