Grumbler is the first boss of 99X, being one of Mr. Crumb's three servants along with Thorn Thing and Digger. Grumbler is very slow and very heavy, however once he catches his prey, there is no escape for them. His main attack is a powerful kick given with his right leg. The mummy was inspired by Gossamer, a monstrous Looney Tunes character.
Grumbler reappears in On the Run!, added as a henchman in Season 6: Mr. Crumb's Dimension of Screams. Much like the Zombots before him, he attacks by sending smaller clones of himself running down the path towards the player, with the main difference being that Grumbler's clones aren't restricted to one lane. Like all henchmen, he is defeated by avoiding obstacles to reach him and using the portal weapons crafted beforehand to send him back to his own dimension.
The developer team have always seen Grumbler as a character who actually tries to have friends, but his appearance and his strength to embrace people - squeezing them - leads him to have no friends. 
Grumbler is a mummy with yellow eyes, wearing pale blue slippers. Only darkness is visible between the folds of his bandages. In On the Run!, dull purple flesh is visible behind the wrappings, and his slippers are white.