Mario
Mario is the short red plumber that is the main protagonist in the Super Mario series. His role in New Super Mario Bros. Wii is a very big one; in single player mode he is the only character, and in multiplayer mode he is only played by P1.
Luigi
Luigi is Mario's younger twin brother and is the taller green plumber. Luigi is available as one of the other playable characters in the game. He is playable for P2, P3, and P4. He is also the character that does the Super Guide feature to show help for the players that can't cross a level.
Yellow Toad
Yellow Toad is one of the two colorful Toads that follows the Mario Bros. on their adventure. He is one of the other playable characters in the game. He is playable for P2, P3, and P4.
Blue Toad
Blue Toad is one of the two colorful Toads that follows the Mario Bros. on their adventure. He is one of the other playable characters in the game. He is playable for P2, P3, and P4.
Princess Peach
Peach is the "damsel in distress" in the game. She is only seen twice in the game: The Beginning and End.
Bowser
The main antagonist of the game. Though he is the final boss he is not seen until the end of the game.
Bowser, Jr.
Bowser's kid. Despite his youth, ever since he first appeared alongside his dad at Delfino Island, he's been a chip off the old shell. These days, Junior's in charge of Bowser's airship armada and is usually seen on the airships carrying Peach away to the next world. Starting at the end of World 4 and every two worlds thereafter, Mario gets a chance to get on board and challenge Junior himself.
The Koopalings
Seven bratty Koopa kids that Bowser's been raising since the days of Super Mario Bros. 3. They hadn't been seen since Super Mario World, but now they're back and badder than ever. Each has their own unique attitude. Larry is the runt of the litter, Roy's the big bully, Lemmy confuses people with his clown antics, Wendy is the spoiled sole girl of the group, Iggy's the geek in the glasses, Morton is the hyperactive loud-mouth, and Ludwig is the oldest.
Kamek
The old head of the mystic Magikoopas, she's overseen Bowser's upbringing ever since he was in diapers. One of his most loyal servants, she personally takes charge at each castle, casting spells to stack the odds in the Koopalings' favor. She then takes personal charge of Bowser's defense in World 8. She's been around a long time so is full of magical tricks. You never know just what kinds of things the room will do once she weaves her spells.