Play as Red Eye
On the character select screen, enter [Right], [Right], [Left], [Left], [Up], [Down], [Up], [Down], [Up], [Down], [Start]. If done correctly, you should hear Red Eye's laugh. After hearing the laugh, press any attack button. Holding [Start] while selecting Red Eye will get his 2P outfit. Instead of the character your cursor is on being selected, Red Eye and his portrait will appear.