Beat the game once to unlock the "Extra Costume" option. Now enter the following codes to get the costumes. These can be equipped during play.
HappyBirthDay Heather Shirt I_Love_You Onsen PutHere2FeelJoy Block Head TOUCH_MY_HEART Don't Touch GangsterGirl God Of Thunder LightToFuture The Light ShogyouMujou Transience BlueRobbieWin Killer Rabbit 01_03_08_11_12 Royal Flush cockadooodledoo Golden Rooster PrincessHeart Transform Costume Shut_your_mouth Zipper sLmLdGhSmKfBfH Play-shirt ProTip Gamepro EGMpretaporter EGM GMRownzjoo GMR SH3_OPiuM OPM badical PSM
Douglas in his Underpants:
Beat the game once to unlock "Extra New Game." At the Title screen highlight "Extra New Game" and press Up, Up, Down, Down, Left, Right, Left, Right, Circle, X. Now choose a difficulty level and start the game.
After beating the game, when playing through a second time there are a few interesting weapons to be found. A submachine gun down an alleyway upon exiting the bathroom, a beam saber in the 2nd floor of the shopping center, and a flame thrower on the 2nd store of the bakery all await you.
Press any trigger button at the Options menu.
Thanks to Revolution readers Lennon, Mark Frick, Bonzy and Andrew!