Smithy is the final boss in Super Mario RPG. At first, he started his invasion by having the only link from his dimension to the Mushroom Kingdom, Exor, crash into Bowser's Keep, shattering the Star Road in the process. He starts to become aware of the threat of Mario and his friends when some of his minions, like a couple of Shysters and an Aero, witness the fall of Mack and Bowyer at certain parts of the game. By the fifth Star Piece, Smithy has already sent his top lieutenants to find the rest of the star pieces, sending Yaridovich to Seaside Town and the Axem Rangers to Barrel Volcano to recover the other star pieces. It is unknown what reason he has to rule Mario's world, other than that he wants to make a world filled with weapons.
Smithy is in possession of the last star piece, which presumably gives him the power to use special magic like "Meteor Shower" for example. It is unknown whether he had it all along or if one of Smithy's henchmen was successful in recovering it and bringing it to him.
He is also stubborn in a sense, going as far as slamming against the ground in a fit of rage. These blind fits of rage get him into trouble, so much trouble that he ends up breaking the foundation for the area he works in, which was only recently built by several of his minions. He also does not take suggestions from anyone, even his own minions, when he was told to think of his blood pressure, and not to shake the floor. Smithy also has a true form, that of a head shapeshifting megalomaniac.
In his true form, Smithy is able to hammer his head into different shapes, including a tank, a hooded sorcerer, a Treasure Chest, and even a protective head. Each head has a different weakness, and they also have different methods of attack. Also, Smithy has two turns to attack, however they are shared with his body and head.
When Smithy takes enough damage, the inside of his body is unable to take the pressure caused by the assault by Mario and friends, and explodes to nothingness, leaving behind the final star piece of the game.