Champion costumes:
     Beat the game in championship mode to unlock the champion costumes.

Extra characters:
To unlock...                ...you have to: 
Freak E. Deke               Beat the game in arcade mode. 
Michael Jackson             Beat the game in arcade mode twice. 
G.C. Thunder                Beat the game in arcade mode three times. 
Wild "Stubby" Corley        Beat the game in arcade mode four times. 
Shaquille O'Neal            Beat the game in arcade mode five times. 
Freedom Brock               Beat the game in arcade mode six times. 
Rocket Samchay              Beat the game in arcade mode seven times. 
Robox Rese-4                Beat the game in arcade mode eight times. 
Mr. President               Beat the game in arcade mode nine times. 
The First Lady              Beat the game in arcade mode ten times. 

Hidden costumes:
Set the system date to...  ...to unlock: 
January 1                   Joey T's New Year's costume. 
March 17                    Referee's St. Patrick's Day costume. 
February 14                 Lulu Valentine's Valentine's Day costume. 
April 23, 2001              Mama Tua's Easter costume. 
July 4                      G.C. Thunder's Independence Day costume. 
October 31                  J.R. Flurry's Halloween costume. 
December 25                 Selene Strike's & Rumble Man's Christmas costume.