The characters involved in the game are as follows.
Mario is back in his third adventure as the main protagonist of Super Mario Brothers 3! Mario sets off on another adventure to rescue Princess Toadstool from the King of Koopas Bowser and his seven malicious koopa kids! Mario will have to fight his way through 8 dangerous worlds to rescue to princess so that he can get some from her...er...I mean so that the Mushroom Kingdom can live in peace and harmony again.
Luigi is Mario's brother and sidekick. Like any faithful companion, he is always at Mario's side whenever Mario needs him to be there. Luigi is the green version of Mario the player number two controls. He is taller than Mario by quite a bit, and he can jump higher than Mario as well. He doesn't get as much credit as he deserves to get.
Later renamed to Princess Peach. Toadstool is one of the main reasons that the Mario series exists and remains popular to this day. Bowser kidnaps her once more in SMB and it's Mario & Luigi who come in and battle their way through 8 worlds to rescue her! She does not make any appearance throughout the game except for the very ending but after each world, you receive a letter from her containing a special item....at least it's not an invitation for cake.
The King of Koopas himself! Bowser makes his return again as the big bad villain of the game but this time he's not alone! This time, he has the help of his seven kids! Larry Koopa, Morton Koopa Jr., Wendy O. Koopa, Roy Koopa, Lemmy Koopa. Iggy Koopa, and Ludwig Von Koopa all make their first appearance in the Mario Bros. series by guarding a castle in each world! Bowsers evil plot is to take over the Mushroom Kingdom but decided to kidnap Princess Toadstool while he was at it.