Star Gladiator 2

Play as Kaede
At the character selection screen, highlight Rain, then press Down(x2), Left(x2), Up.

Play as Rai-On
At the character selection screen, highlight Byakko, then press Up, Down, Left(x2), Right, Left, Up.

Play as Evil Gamof
At the character selection screen, highlight Gamof, then hold Start and press A.

To Play as Inferno on Soul Calibur, select Xiangua and use her 3rd
outfit and finish the game. After the credits Inferno will now be a playable character.

Adjust Replay Camera
During a replay, press B to point the camera to the loser

More Stages & Characters
You will get a new stage or character for each character you use to beat the game.
The last character you get will be Edge Master

Arcade Mode
Using every character, complete the game (including the bonus stages)

Extra Costumes
At the Character Select screen, press Y while selecting your fighter

Play As Unknown Soul
Get Edge Master (scroll up to "More Stages & Characters"), then at the Character Select screen, hold R while choosing your fighter to get a transparent version

Extra Weapons
Get Edge Master (scroll up to "More Stages & Characters"), then complete all mission battles. Now at the Character Select screen, hold L while choosing your fighter