Mario and Luigi were different. That was fine by them. That just meant they each had their own strengths that suited them.

Mario was the adventurer- always saving a princess, fighting Bowser, helping a Yoshi or Toad. It's what he did. And Luigi did what he did- taking care of the house, managing money, giving first aid, whatever he could with out actually being in any danger.

Mario was the star of the show while Luigi kept it all together behind the scenes. They liked it that way. Mario would never not help someone in need, and Luigi would give anyone in the world moral support. That was how they were useful.

Of course, Mario would always be the guy who would force fun upon Luigi, or anyone in general- the more dangerous and thrilling, the better. And Luigi would always stick to the sidelines as much as possible, the voice of reason. But that was how things worked out.

The two plumbers were close brothers and the best of friends. But every once and a while Peach would be woken up by screams, shouts and multiple other forms of chaos when the two brothers got into a fight and tried to knock each others' heads off. This didn't happen often considering Mario's heroic nature and Luigi's distaste for conflict. And more often than not, they'd ban together and fight a common enemy, rather than beating each other. They really were stronger together, than seperate.

Surprisingly, on some levels, the biggest difference between Mario and Luigi was actually Peach and Daisy. To be more exact, the relationships between Mario and Peach, and Luigi and Daisy.

Mario treated Peach like princess (despite the fact she was), a goddess, he would die for her every time and would only keep pursuing her if she were ever to get bored of his presence. Every time he would go to her rescue if she got kidnapped or attacked or whatever else happened to the princess with insanely, impossibly bad luck. Mario could not say no to Peach. He would give her anything she wanted and more. And Mario would always keep Peach off the battlefield. Peach would not fight as long as Mario had a say in things. Mario would give Peach anything and take nothing in return.

With Luigi, it was different. Daisy was a person, a very dear person, but not a goddess. And Luigi would live for Daisy. He would live for her in any way that could be perceived. He would live solely for her benefit, but, maybe even more importantly, get to the brink of death, hopeless case, and still pull through. Just for Daisy. Daisy, with no crazed turtles after her, would more likely than the opposite, be the one to save Luigi, a fact that never ceases to amuse Mario. But above all else, Luigi and Daisy fought together. They'd stand on the front line as equals. Luigi was so alive when around Daisy. She could get him try anything, and he could get her to be good. Give and take.

Mario and Luigi were different, a fact preferred by both.