If you ever wanted to know why Luigi is thinner than Mario or how he bears living in the show of his older brother, this exhaustively researched article over on GamesRadar's got you covered.
"Luigi: A life in the shadows" covers the other brother's origins from palette swap to star of his own (two) video games. Anybody else remember playing Mario is Missing? Yeah, that game starred Luigi.
Best bit? Right here...
But Luigi's heart has strayed at least once during the previously mentioned Mario & Luigi: Superstar Saga. The Beanbean Kingdom the two stars explore has a prince to match the Mushroom Kingdom's Princess Peach. Prince Peasley is a dashing (if self-absorbed) pretty boy, who throughout the game slowly gains Luigi's attention. As time goes on, Luigi seems almost enamored with Peasley, blushing at his compliments and eventually trying to hug him, which Peasley deftly dodges, as Peasley seems to only love himself. But this bi-curious side of Luigi has yet to return, as he's since stayed pretty true to Daisy. However, that hasn't stopped the devs of the M&L series from continuing to ramp up Luigi's effeminacy to Tingle levels of fab.
I totally did not know that. I thought when Luigi wasn't with Daisy, he was letting Peach cheat on Mario with him. Also, I thought John Leguizamo and Samantha Mathis as the "ideal" Luigi/Daisy couple.
Check it out, if you're a little Luigi-curious:
Luigi: A life in the shadows [GamesRadar]