Captain Viridian, misspelled in Super Meat Boy as "Capitan Viridian", is from the indie platformer VVVVVV. He's an unlockable character for the PC/Mac/Linux versions of the game. The requirement to unlock him is to obtain 90 bandages.
He has the ability to flip gravity, but only when he is on stable, horizontal ground. This can make it easy for him to travel long, vertical, corridor-like levels such as 3-6 (The Shaft).
Con: Can't run very fast and has no control over the height of jumps when gravity is flipped.
Description: "VVVVVV VVVVVV"