Old Man Grumpus is old and grumpy. According to Snowman Willie in Great and Grumpy Holiday, the reason he is so grumpy lies in his past. Every year at holiday time, since he was very young, he has always gotten pairs of long john underwear for his present. He never got what he really wanted, a choo-choo train. Even in Focus Wubbzy, the package that Wubbzy delivers to Mr. Grumpus contains three pairs of long johns.
Old Man Grumpus lives up at the top of Mount Zubba Bubba. He is very protective of his property. When others climb the mountain, he tries to scare them off.
When Old Man Grumpus is not busy being grumpy, he's rather nice. After Wubbzy finds his mother's Rooty Tooty Fruitcake in his present, Mr. Grumpus shares the cake with Wubbzy and his friends. When Mr. Grumpus finds that the people of Wuzzleburg gave him the gift he's always wanted, a train, he feels bad for how he treated them in the past and redecorates the Christmas tree (of which he'd taken the decorations off). He also gives them "presents for everyone" (holding up two pairs of long johns) and a ride on his new train.
Old Man Grumpus has light blue skin with a light grey beard and a missing tooth. He has black eyes and a square-shaped nose. He wears pinkish red long johns and a green hunting cap with ear flaps and a tan colored fold with a grid on the front of it. He also wears turquoise colored gloves and dark grey boots. His back is hunched and his belly sticks out in front of him a little bit.
In Great and Grumpy Holiday, a flashback scene occurs in which Old Man Grumpus is shown as a toddler, "Little Grumpus", as a kid, "Grumpus", and then as an old man.
He could be similar to the Grinch from Dr. Seuss' How the Grinch Stole Christmas.