Luigi, who is Mario's brother and also a heroic figure in the Mushroom Kingdom, is easily distinguishable with his characteristic green hat and green shirt.
He is a kind-hearted individual, though he can get a bit jittery, particularly when it comes to ghosts. Nevertheless, his capabilities are equivalent to those of Mario. So when these two brothers join forces, there's absolutely nothing they can't accomplish.
Luigi is taller than Mario and can jump even higher. If you observe carefully, you'll likely also spot that the shape of his moustache is somewhat different too.