Pixls are basically taking the place of "partners" in the other Paper Mario games.
Tippi is the pixl that you first start off with in the game She is able to find hidden objects by the Wii remote being pointed at the screen. As well when you do that on an enemy or object, she gives a description of that object!
Found in chapter 3-2, Thudley enables you to do a ground pound which helps cause high amount of damage and other things.
You find Thoreau early in the first chapter. Thoreau's ability is to be able to pick up objects and throw them, including enemies. So if you can't reach a blue block, look for an item to pick up and throw at it courtesy of Thoreau.
Slim is found in chapter 2-3 and as the name say, he allows to turn sideways and become paper thin. Which will help when you need to slip through bars or even cracks in the walls.
Piccolo is another sidequest pixl. The way to get Piccolo is you need to do Merlee's Request and find him in a Random Locked House. Piccolo's ability is it plays the characters theme music and changes a few of it's sound effects. Oh and cures status ailments.
Found in chapter 4-2...in a bathroom. Fleep helps by flipping an area when it looks like something could be on the other side.
Chapter 5-4 is when you get Dottie and well Dottie allows you to shrink to a smaller size to fit through passages and visit the Itty-Bitty shops. As well enemies will not be able to see you.
Dashell is a optional pixl and the way to get Dashell is to beat Flipside's Pit of 100 Trials. Dashell's ability is to make the player run twice as fast.
Found in Chapter 5-2. He allows you to use a hammer to destroy obstacles or your enemies.
Found in 3-4. Carrie does just what his name says...carry you on a floating platform to get you around. She can come in handy when your going over water, and spike hazards.
Found in the early going on the 2nd chapter. Boomer is a Pixl that blows up and comes back. Kind of like Bombette. Just set Boomer in place and let him do his magic!
Is one of the optional Pixls you can get. He can be found in Chapter 3-1 and what his ability is to make a spike barrier around you.