In the character select screen, rapidly do this combination: up, up, up, up, down, down, down, left, left, right. If done correctly, Abubo will appear in the middle of all characters.
In versus mode, after you have selected your characters, hold all the buttons (ABCD) until the round starts. NOTE: Johann will mimic the colors of the character you selected, so anything other than the default color will result in a nasty appearance.