Baby Characters
Baby versions of many characters become available if you beat specific teams or complete challenges in the main adventure mode. For example, you can use Yoshi to get Baby Donkey Kong out of a palm tree in DK's Jungle or you can get him by beating Donkey Kong's team. On the other hand, Baby Luigi is unlocked if you pull all the levers in Mario's Stadium to connect the bridge. Baby Luigi can be used to defeat Bowser Junior's team and unlock Baby Mario.
Star Characters
To unlock the powerful star version of each character you must first become a member of the Secret Shop. Select Wario as your character and head to DK's Stadium. Use Wario's ability to open the chest here, granting you access to the secret shop. Once here, use credits with any selected character to first purchase star candy, then purchase three sets of Star Candy (X2) to unlock the star version of character.
Shy Guys
The Shy Guys are the ghosts that populate Yoshi Park, and all of them can be unlocked and rendered playable. Red Shy Guy is unlocked by shooting the Yoshi Park piranha plants. Two more shy guys can be found near the ferris wheel at the entrance to the park. Use Peach to retrieve Green Shy Guy from the log. Two more can be found under trees near the stadium.
Error Items
Any character can use an error item to increase his offensive abilities. Select Wario as your playable character and go to Wario City. Open the chest here to unlock the fire ball error item. The Mini Boos item is in the chest in Mario Stadium. Use Wario to open the chest in Peach's Ice Garden to get the last error item, the POW Ball.