Bandage girl has a random chance to glitch every time you spawn on a level. She can glitch in any level from 1 to 20 for each of the worlds 1 to 6, whether you're in the light or dark half of a world. There is exactly one negative level for every world.
To unlock level -1 for a world, pick a level in the world that is short and relatively easy. Below are good examples (in the light world) including a recommended character:
- 1-3 (The Gap) using anyone
- 2-10 (Johnny's Cage) using Jill (if you don't have Jill, see this video)
- 3-6 (The Shaft) using anyone
- 4-6 (Leviathan) using anyone
- 5-10 (Grinding Mill) using anyone
- 6-2 (Schism) using anyone
Then:
- Look at or get to bandage girl and see if she is glitching. She will be spinning and making a distinctive pinging sound. I recommended levels where you can (mostly) see her from spawn.
- If she is glitching then touch her to unlock level -1. Otherwise, kill yourself, respawn, and go back to step 1.
- To play the level you just unlocked, go the light half of the world you unlocked it in. It will be available to play as a yellow-colored level beside level 1.
There are 19 for Steam on Windows/Mac OS (15(?) for XBox Live Arcade, different exclusives).
The Wikipedia page has a list of which characters are available on which platforms, and their origin.
To quote:
These characters can be unlocked by collecting enough bandages or completing certain warp zones.
There is a fairly solid list of how to get pretty much everything here.
Summarised:
Character Origin # Bandages
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Head Crab Half-Life 10
Josef (Robot) Machinarium 30
8Bit Meat Boy 40
Naiji Aquaria 50
4Bit Meat Boy 60
RunMan RunMan Race Around the World 70
4Color Meat Boy 80
Captain Veridian VVVVVV 90
Steve (Mr. Minecraft) Minecraft 100
Character Origin Warp Zone
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Commander Video BIT.TRIP RUNNER 1-12
Jill Mighty Jill-Off 2-8
Ogmo Jumper 3-16
Flywrench Flywrench 4-18
The Kid I Want to be the Guy 5-7
Character Available
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Bandage Girl Once you beat the main game, only in The Cotton Alley world
Meat Ninja Once you 100% the game
Character Character Select code Activate by selecting
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Super Tofu Boy petaphile Meat Boy
Goo Ball ballgoo ???
Brownie RB,RB,RB,B,B,B,X Meat Boy
To unlock on the Character Select screen, just type in the listed code on the selection screen which appears when choosing a stage.
Super Tofu Boy was put in following the PETA incident.
According to this, activating Brownie on the Character Select screen requires an XBox 360 controller.
Teh Internets Chapter is also unlocked by collecting bandages (20x) and is an online chapter which contains bonus levels.
Best Answer
You need to complete 17 levels in each world to unlock the boss. From the Super Meat Boy Blog:
The exception to this rule is the 6th world (The End), which requires all 5 light world levels to be completed to unlock the boss. This world also has a dark world boss, which requires 85 of the dark world levels to be completed to unlock (source).