Luigi

Luigi is a green-clad plumber and Mario's younger (albeit taller) brother. He is an unlockable character in Super Smash Flash and will most likely be unlockable in Super Smash Flash 2.

In Super Smash Flash
Luigi appears as an unlockable character that can be unlock by completing the Adventure Mode in normal, hard or very hard level with Mario without continuing.

In Super Smash Flash 2
Luigi once again appears in SSF2 along with Mario, Peach and Bowser. His moveset will be similar to the SSB series, and more likely than not be a hidden character