Switch Heads
Go to the Character Select Screen. Highlight each of the following characters in the order in which they appear and press start after each one: Al-Rashid, Takeshi, Mordos Kull, Xiao Long, and Namira. The players you fight with will switch heads.

Bunny Slippers
Go to the Character Select Screen. Highlight each of the following characters in the order in which they appear and press start after each one: Ragnar, Dregan, and Koyasha.

Two-Player Practice
At the Title screen, press Start on both controllers at the same time. Play as War Mech and Ichiro On the Copyright screen, rotate the Control Pad counter-clockwise until you hear a tone.

Play on Various Stages
Go to the Character Select Screen. Highlight each of the following characters in the order in which they appear and press start after each one in order to play on their respective courses:
Machu Pichu
Namira, Koyasha, and Taria
Golf Course
Koyasha, Mordus Kull, and Takeshi
Castle
Mordus Kull, Taria, and Ragnar

Battle a Random AI
Go to the Character Select Screen. Highlight each of the following characters in the order in which they appear and press start after each one: Hell Knight, Xiao Long, Dregan, Namira.

Big Head Mode
Go to the Character Select Screen. Highlight each of the following characters in the order in which they appear and press start after each one: Ragnar, Al' Rashid, and Takeshi.

Play as Grendal
One player must win three games in a row in two-player mode. Go to the Character Select screen, highlight Executioner and hold Start. Executioner will turn into Grendal.

Play as Pojo
Perform Taria's Execution. At the Character Select screen, hold start. Taria will turn into Pojo the Chicken.

Select Character Stages
Go to the Character Select screen. Highlight the character whose stage you want to play on. Press Start four times.

Small Fighters
Go to the Character Select Screen. Highlight each of the following characters in the order in which they appear and press start after each one: Takeshi, Al' Rashid, Ragnar and Xiao Long.

Change Colors
Press either L or any of the C buttons. When you find the color you wish to be, hold down the respecive button and then press A.

Thanks to RCreviston for sending in this code!
Play as Ichiro and Gar
On the Copyright screen press on the anolog stick "forword, up, back, down, forward, up, back, down" in other words rotate the stick. Then on the player select screen Ichiro and Gar will be at the top.