Ribbit King Cheats

Pan-Pan and Black Alien:
After beating Picwick for first time you will unlock Pan-Pan and the Black Alien in versus mode.

Lunk:
After beating Pan-Pan frolf against and defeating Lunk you will unlock him in versus mode.
User Cheat by GreenEyes

Pappy:
After beating Lunk then go back to Ribbittron and beat Pappy you will unlock him in versus mode.
User Cheat by GreenEyes

Sir Waddlesworth:
After beating Pappy to frolf against Sir Waddlesworth. By winning the match you will unlock him in versus mode.

Unlocking clips:
By collecting 10, 20, or 30 Bottle Caps or complete any other task on the bonus disc.