Undergrounder is a hard-working and conscientious Psymonster who is always trying to get extremely prosperous with some constructive scheme or another. If he is not digging for gold out in the wilderness of the Psymonster World, then he is hoping to strike oil with his pickaxe, or find buried treasure, or uncover a crashed flying saucer that he can sell to a museum. None of his plans ever pan out, but he just comes up with a new one and heads right back out into the wilderness to try his luck again. Some may prefer being a metropolitan Psymonster with a roof over the heads, but that is no life for Undergrounder. Any Psymonster must give him a sky full of stars, a campfire, and a tin of beans, and he is as happy as a prairie dog in dirt. It could be possible that he will do with all of that money when he finds his fortune. It could also be so easy that he will get a big sack of fluorescent and vivacious minerals, maybe eventually puts on his gloves, and buy himself the biggest tin of beans in the Psymonster World.
He is a chestnut rabbit with hazel eyes, a pink nose, a yellow miner hat, a vest, and shoes, an orange shirt, gray goggles, a black backpack, and blue pants.
- The name "Undergrounder" is probably derived from the word "underground", meaning beneath the surface of the ground.
- Undergrounder is one of the few Psymonsters to have "Under" in his name (others include Undersleep and Underman).