Play as Pojo:
Pick Taria and do her Execution. At the next Character Select screen hold Start and Press an attack buttton to choose Pojo.

Hidden Characters (for 2 player mode):
Grendal:
Win three 2-player games. On the fourth 2-player game. Go to the Executioner, Hold Start and press attack. (Be sure to keep holding Start until the fight starts).

War Mech and Ichiro:
At the copyright screen (the very first screen) press right, up, left, down, right, up, left, down. War Mech and Ichiro appear above The Executioner and Lord Deimos.

Play as Ned:
At the Character Select screen move the cursor and press start on Koyasha, Executioner, and Lord Deimos. Pick any fighter and press A or B.

To unlock the secrets below, move to each character listed and press start after each one, then go to your desired character and select as usual. All of these secrets are for two-player mode only:
Big Head mode = Ragnar, Al' Rashid, Takeshi
Midget mode = Takeshi, Al' Rashid, Ragnar, Xiao Long
Mini Golf Course stage = Koyasha, Mordus Kull, Takeshi
Inside Castle stage = Mordus Kull, Taria, Ragnar

To select a specific stage to fight in, go to the character's portrait on the player select screen and press Start four times. Then choose your character as usual.
To perform Pojo's execution = Push forward then kick.

Different Colors = At the Character Select screen, while on the character you want to choose hold any of the C buttons for different colors.

Play in Bunny Slippers = At the Character Select screen press start on Ragnar, Dregan, and Koyasha. Choose your fighter then press A or B. Nice shoes.