Alternate costume colors:

Press the various Punch and Kick buttons while choosing a fighter
on the character selection screen.

Fight as Inoz:

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

Fight as Darkness:

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

Extra options:

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