In the character selection screen you have to type in the code "ballgoo" and then you select a character (any will do) and then your character will be the goo ball from world of goo.
Submitted By minnyman67
Xbox 360
Unlockable Characters
4 Color Meat Boy: Collect 80 bandages
4Bit Meat Boy: Collect 60 bandages
8Bit Meat Boy: Collect 40 bandages
Alien Hominid: Collect 30 bandages
Commander Video(Bit Trip): Warp Zone in World 1
Flywrench: Complete The Fly Guy! Warp zone in 4-18
Gish: Collect 10 bandages
Jill (Mighty Jill Off): Warp Zone in World 2
Meat Ninja Achieve: 100% completion
Ninja (N+): Collect 100 bandages
Ogmo (Jumper): Complete The Jump Man! Warp zone in 3-16