Accumulate over two hours of total game play. Then at the character selection screen, hold L while choosing Mamazo.

Accumulate over five hours of total game play. Then, Darkness will be selectable at the character selection screen.

Accumulate over ten hours of total game play. Then, an "Extra options" selection will appear on the options menu.

At the character selection screen, press the various Punch and Kick buttons while choosing a fighter.