Metal Mario is one of Mario's many rivals. he is a member of the mafia.
One day Mario discovered a new metal power-up. After a few minutes, the metal became a liquid and crawled off of him. It took a form that looked like Mario and began to terrorize the world.
Metal Mario saw that Mario had a partner: Luigi. Metal Mario decided that he needed a partner as well. He took a piece of metal out of his body, and the piece turned into Metal Luigi. The Metal Mario Bros have been working together ever since.