Luigi is Mario's brother and the co-protagonist of the Mario franchise. This time, however, he's one of the main antagonists.
After his brother ruined the Mushroom Kingdom, he aided the Princess and the denizens of the kingdom in coming up with a plan to get back at him.
Luigi is never seen outside of the intro until after Mario reaches the Mushroom Kingdom, thus his Brutal Mario incarnation is quite the mystery, as Carol still hasn't finished that World.
Luigi has a similar appearance to his older brother. The main differences are that he's thinner, slightly taller, wears green instead of red, and has a slightly different moustache.
While Mario is your standard silent hero, Luigi is quite the chicken. He tries his hardest, but he's rather easy to frighten. Regardless, he's still someone you can rely on. He even mustered up the strength to save his brother from King Boo! Twice, in fact!
He also saved him once from Bowser... But that was in a non-canon edutainment game.