Use the mosters in the cities listed below to unlock the other monsters. There may be more then one monster to unlock the secret ones. And you might have to re-do the city to unlock the other ones that are in it.

Leon - Play as any starting monster in Las Vegas
Kingston - Play as Lizzie in Las Vegas
Jack - Play as Harry in Las Vegas
Marco - Play as Ralph in San Francisco
Shelby - Play as Lizzie in San Francisco
Natalie - Play as Marco in San Francisco
Kyle - Play as Kingston in London
Icky - Play as Ralph in London
Edwin - Play as Wally in London
Cal - Play as Kyle in Los Angeles
Amanda - Play as Kyle in Los Angeles
Bart - Play as Kingston in Los Angeles
Rojo - Play as Kyle in Chicago
Harry - Play as Marco in Chicago
Nick - Play as Harry in Chicago
Croc - Play as Rojo in Hong Kong
Venus - Play as Croc in Hong Kong
Jill - Play as Amanda in Hong Kong
Wally - Play as Marco in New York
Rocky - Play as Cal in New York
Joe - Play as Natalie in New York
Philbert - Complete all the Par Point Stars in all the cities.
Eyegore - Complete all the Challenge Stars in all the cities
Bubba - Unlock all the monsters, secret and normal.