Mutant Mudds Super Challenge features 20 playable characters that can be unlocked, each from their own unique game series (with a few exceptions). All characters are found in the VB/GB levels found in each regular level. To obtain each character, you must find a hidden pathway and press up once you are through. From there you just need to shoot the wall blocking the character and touch the bubble to unlock them.

Keep in mind that you must actually clear the level in order to unlock them. Also take note that if you get the character and die before reaching the checkpoint or the end of the level, you'll have to get them again.

To switch to a character, go to the title screen and press the start button on your save file to bring up the character select screen.

Game of Origin: SteamWorld Dig.
Level: 1-1 Bonus level.
Where to Find: While playing through the bonus level, you'll eventually find a jump platform that will take you to the back plane. The door that leads to Rusty is hidden underneath it, so approach it from the left side.











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Game of Origin: Treasurenauts[www.renegadekid.com].
Level: 1-2 Bonus level.
Where to Find: To the left of the checkpoint is a platform with a spike. The door to Smith is undernear that platform (approach from the right).












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Game of Origin: Noitu Love 2: Devolution.
Level: 1-3 Bonus level.
Where to Find: On the right side of a wall with a spike on the bottom (the second before the jump point).
















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Game of Origin: Bomb Monkey[www.renegadekid.com].
Level: 1-4 Bonus level.
Where to Find: About halfway through the level there is a pillar in the back plane with two spikes on the top. The door is on the left side of it.

Game of Origin: Woah Dave!
Level: 2-1 Bonus level.
Where to Find: On the right side of a pillar with a spike on the bottom, shortly before the end of the level.










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Game of Origin: ATV Wild Ride[www.renegadekid.com].
Level: 2-2 Bonus level.
Where to Find: On the right side of the first platform with the hammer on the top, right after the jump pad.















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Game of Origin: Mercenary Kings
Level: 2-3 Bonus level.
Where to Find: When you first jump into the background, boost up and keep going left on the wall until you eventually come across the entrance.













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Game of Origin: Treasurenauts.
Level: 2-4 Bonus level.
Where to Find: On the right side of the platform before the jump pad that is in the back plane.

Game of Origin: Adventures of Pip.
Level: 3-1 Bonus level.
Where to Find: Go into the hole that's to the left of the checkpoint, and press up when you're at the end.












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Game of Origin: Original character from this game.
Level: 3-2 Bonus level.
Where to Find: Through the wall to the right of the checkpoint.










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Game of Origin: Bit. Trip series.
Level: 3-3 Bonus level.
Where to Find: On the right side of the wall next to the jump pad.










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Origin: Creator of the website GoNintendo[gonintendo.com].
Level: 3-4 Bonus level.
Where to Find: On the left side of the jump pad, right before the checkpoint.

Game of Origin: Retro City Rampage.
Level: 4-1 Bonus level.
Where to Find: On the left side of the platform to the left of the checkpoint.












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Game of Origin: Original character from this game.
Level: 4-2 Bonus level.
Where to Find: On the left side of the platform to the left of the checkpoint.










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Game of Origin: Teslagrad.
Level: 4-3 Bonus level.
Where to Find: On the left side of the platform with the jump pad on top of it, right before going into the back plane.













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Game of Origin: Oddworld series.
Level: 4-4 Bonus level.
Where to Find: On the left side of the second platform before the goal point.

Game of Origin: VVVVVV.
Level: 4-1 Bonus level.
Where to Find: On the left side of wall above the checkpoint.













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Game of Origin: Shovel Knight.
Level: 4-2 Bonus level.
Where to Find: On the right side of the platform under a bunch of fall-through platforms.













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Game of Origin: Shantae series.
Level: Bonus level.
Where to Find: On the right side of the third spike-swinging platform in the back plane, the one right before the checkpoint area.















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Game of Origin: Xeodrifter.
Level: Bonus level.
Where to Find: On the left side of the platform with the sword mudd in the back plane (the one right after you have to hop on 5 vanishing platforms shortly before the checkpoint).

I hope this guide helped in finding these characters. If you feel like this guide could be improved, please say so in the comments.