The Yellow Miner Girl, Purple Pirate Girl, Green Mariachi Man and Blue Bear Suit Man all randomly appear in coffins throughout the 4 main areas. They are limited to ONE per area and they are RANDOM i.e. if you find the Yellow Miner Girl in the Jungle, you can no longer find a coffin in the Jungle.
Black Van Helsing - Haunted Castle, at the top of the castle in a coffin
Blue Bear Suit Man - Coffin (Mines/Jungle/Ice Cave/Temple)
Green Mariachi Man - Coffin (Mines/Jungle/Ice Cave/Temple)
Leopard Suit Man - Beat the game (shortcuts allowed)
Meat Boy - The Worm, at the bottom in a goo sac
Purple Pirate Girl - Coffin (Mines/Jungle/Ice Cave/Temple)
Yang - Beat Hell
Yellow Miner Girl - Coffin (Mines/Jungle/Ice Cave/Temple)
Free items without angering shopkeepers
When playing with a friend (or just using another controller) keep playing until you come to a shopkeeper. Have the second player (person without the flag) pick it up. Then using the main character (person with the flag) proceed to finish the level while the other player sits in the shop holding the desired item. You do have to beat the level before the person's time runs out or he will die and you won't get the item. Also if you pick up any item that you can hold onto (jetpack, cape, mattock) it'll duplicate and both you and the other player can have that item. All other items will go to the player that held them in the shop. Simply kill them off and take them for yourself if you so choose.