Complete Quest with each character (includes Player 2).

Character	How to unlock
Boogie	Complete Quest With Obake.
Chassey Blue	Complete Quest With Astronaut Bob O., Garbage Man And Agent R. Chase.
Convoy	Complete Quest With Houston.
Dave's Cultsmen	Complete Quest With Convoy.
Dusty 'Earth'	Complete Quest With Padre Destino.
Houston	Complete Quest With Sheila, John Torque And Flying All Star Trio.
Lord Clyde	Complete Quest With Dallas 13, Nina Loco And Molo.
Obake	Complete Quest With Lord Clyde.
Padre Destino	Complete Quest With Chassey Blue.