Mario
Mario is the first playable character in Super Paper Mario. After meeting Bestovius, Mario obtains the ability to switch from 2-D to 3-D by pressing the A button. Being in 3-D can let Mario discover hidden doors, pipes, and items. But Mario can only be in 3-D until his meter drains. If his meter drains, Mario will lose one HP point and the meter will start over.
Princess Peach
Princess Peach is the second playable character in Super Paper Mario. Mario finds her falling out of the sky because she mysteriously teleported out of Castle Bleck. Since she was unconscious, Mario has to get a hot soup. Peach can glide with her parasol by holding down on the 2 button.
bowser
Bowser is the third playable character in Super Paper Mario. Bowser is found in the Bitlands in his new fortress. Before unlocking him, you must first battle him. After that, Mario and Peach ask him to be on their team. After a long discussion, he finally agrees. Bowser can breathe fire and does double damage to any enemy.
Luigi
Luigi is the fourth and last playable character in Super Paper Mario. Up until after Sammer's Kingdom, Luigi was brainwashed into the Mysterious Mr. L. You must fight him twice in the game: once in Outer Space in the Whoa Zone and once in the blank nothingness of what was left of Sammer's Kingdom. Mr. L has a giant machine called the Brobot that is noticeably powerful and is fought both times with Mr. L. Mario finds him huddled up in a corner in the Underwhere. Luigi's special ability is to jump very high lengths.