*gasp*Stumpers said:Luigi had a ying-yang relation to Mario. While Mario was steadfast and jumped right into the fray, Luigi was cautious and watched out for his own neck much more. However, both were heroes. Luigi was the brainier of the two as well.
It's one of the connections that the video games have with the show. There are a bunch if you look around -- including those little miniature Mario's that appeared in Super Paper Mario. They first appeared in an episode of Super Mario World as a power-up that came out of a block.