There are 4 playable characters in Super Paper Mario and of course you start off with the hero his self Mario. You will also meet up with Luigi, Peach and Bowser later on in the game.
The savior of the Mushroom Kingdom is back. Not much has changed with Mario from his Paper Mario The Thousand Year Door days. Still got his jumping and his hammer. He is followed around by Pixls and you start off with the plumber in the game. He has the ability to go from 2D to 3D and he's the only character in the game to do it.
Princess Peach is another playable character you get to play as. You get her before the start of Chapter 2. Her ability is she is able to float with her parasol. Meaning if you need to float to a ledge and can't get there by jumping, she may be the best bet to use. As well she can hide under her parasol to shield herself from most attacks.
And Mario's brother is the 4th character in the game. You get him in Chapter 6. Now Luigi's ability is he can crouch down and do a super jump. But if he hits the roof he becomes flat and floats down to the ground but your able to control him!
The King of the Koopas is back and you get to play as him! You get him in Chapter 3. Now Bowser's ability in the game is blowing fire. Who would of ever known? Bowser is the brute strength of the team but is not one to go fast.
 Other Characters
Other characters that have a big role in the game but aren't playable include:
Merlon-Gives advice to the heroes as often as he can.
Merlee-She guarded the third Pure heart and lived in a swamp but after moved to Flopside.
Bestovius-Taught Mario the skill to flip dimensions.
Nolrem-Is the same as Merlon but is the Flopside version.
Jaydes-Queen of the Underwhere, wife of Grambi, the King of Underwhere.
Grambi-King of the Underwhere, husband of Jaydes, the Queen of Underwhere.
Luvbi-Daughter of Grambi and jaydes.