Play as Axel.
Select the Playground Peril level in tournament mode. Look for the swing set next to the merry-go-round and shoot the middle swing with a missile. Now collect the statue of Axel that appears on the screen.
Play as Darkside
Select the Kitchen level. At the sink there's a plumber, drive down his butt and onto his leg and then napalm his butt. A hole will then be blown into the wall. Enter it to collect Darkside.
Play as Mime
Select the Holiday Havoc level in endurance mode. Find the Christmas tree and shoot missiles at it until it's destroyed. Then get on top of the fireplace and jump off backwards to shoot the missile at the stockings. If you destroy all four stockings, Mime will be unlocked.
Play as Trapper
Beat the game with Mime.
Shock Therapy level
In endurance mode, kill 10 enemies.
Buster's Lanes level
In endurance mode, kill 20 enemies.
Holiday Havoc level
In endurance mode, kill 30 guys.
While the game is paused press Square, Circle, Triangle, L1. A sound will confirm the code. To activate slow motion, press L1 + R1 in the air during a railslide.