I always found Mario to be your average clear-cut hero-by-default character; I mean, he doesn't have any real character development. He's just the main character who you end up playing who always has to save the world.
Luigi, on the other hand, is pure joy. He's got pep, personality, and is wears green! And this is just priceless:
"My "part" was...GRASS! I played grass by the side of the road! Grass, Bro! Grass! I had to sprawl out on the stage and keep quiet. Everyone but me had lines... I don't care if I was wearing green! Who casts someone based on that? It was awful! . . . The party only bummed me out more, so I snuck out the back. Outside were tons of fans! MY fans! Fans of grass! . . . I just couldn't believe it! Imagine! Cheering for grass!" ~ Luigi, Paper Mario2
Best quote ever, lol. Amusing and depressing at the same time.