No Luigi is not a playable character in Mario Sunshine sorry!
You can't. He's not unlockable and there are to cheats for playing him.
he is not in there because it is really talking about the story of mario.
No. I don't think so.
Nope. that'b be cool though...
I don't think you can. i have the game, and i tried all kinds of stuff but it just doesn't work!