Here's how to play Sumo Santa in the Special Edition: At the character selection screen press:
A, C-right, A, C-right, C- down, C-up then at the ? box press right shift to scroll to him.
Play as High Five
At the character select screen, press and hold the L button while inputing the following code:
C-up, C-down, C-left, C-right, B, A
If done correctly, you will hear a beep. Now move the cursor over one of the question marks in the lower corners and press the R button. High Five appears! It's Dr. Kiln's right hand that was removed from his evil body! This character is powerful.
Play as Boogerman
On the character select screen, hold Z or L and Press B, B, C-right, C-right, C-left, C-right, then press R on the "?" box to play Boogerman.
Play as Earthworm Jim
Hold Z or L and press B, C-left , C-up , C-right , C-down , C-up. You will now hear a sound. Then press R on the "?" box.