Luigi will be playable in Super Smash Bros. Melee...confirmed straight from the master himself!
In the most recent issue of Computer and Video Games Magazine, Shigeru Miyamoto confirms what we have all suspected: Luigi will be in SSBM!
When asked if Mario and Luigi will appear in a GameCube game together, here's the revealing answer Shiggy gave:
"What we want to avoid is just graphically updated versions of Mario and Luigi. You'll see Mario at the end of Luigi's Mansion, and of course they can fight together in Super Smash Brothers Melee"
