Rolling Stone has put together a great interview with Shigeru Miyamoto on his time with Nintendo, and where he thinks they’re going.
Though he talks about such things as the original Donkey Kong and his creative process, his revelation about Luigi is perhaps the most interesting from the interview:
“There’s something I’ve been meaning to share with somebody,” Miyamoto says with a grin, letting us in on a secret of the development of the original Mario Bros. “One of those constraints was that because of memory limitations, the second character had to be identical to the first character in appearance. And so we looked at that and said, ‘Well even if we have the same character, we could potentially change the color of the character.’ But again we were limited in the color palettes – we didn’t have much in the way of additional colors that we could use. And so we looked at the turtles in that game. Their heads are sort of skin-toned, their shells are green, so what we could do is we could use the color palette from the turtle on this character. And so from those technical limitations we said ‘Okay. We have these two characters. They look the same, other than the fact that their colors are different. Obviously they must be twins.’ From there, we decided, ‘Okay, they’re twins and this other character [Luigi] must be the younger brother.'” – Rolling Stone Interview
More insights, and the full interview, can be found at Rolling Stone.