Go to the elevator and find "O". That will take you to all of the characters. To change, press up on a character.
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Ohhh, that. Find "flashlight" and it will make the level dark. I forgot what Jerald does though, but you can also turn off destroyables and choose what timer you want. Coneball uses the original timer, Sucrose uses the Sucrose Snowstorm timer, and Yogurt is still being worked on. But I don't think Multi-Showdown does anything. It maybe does but I don't know.