@ 1st Q:
Luigi... is Italian. Nay, he's not just Italian, he's the BROTHER of an Italian. They're an Italian
family. Y'know what that means?
... Luigi is the silent mobster. Mario does all the talking, and Luigi feigns his scaredness. He's so good at feigning it, that he even feigns it in the face of ghosts like those of Luigi's Mansion. Heck, he OWNS A MANSION! And he was philanthropic enough to donate it to the brawl community to fight in. He's a philanthropic mobster who can fake cowardliness. Oh, and he's tall.
And he has an awesome 'stache. That alone makes him awesome to play as (and better than Mario - [Luigi's 'stache] > [Mario's 'stache]).
@ 2nd Q:
LUIGI LACKS LIGHTNING! He needs more 'Thunda!' from Mario & Luigi RPG!!! His forward aerial should be, like, a thunderhand chop with a disjointed sphere of electricity swiping over his head and dissipating violently (not necessarily 'exploding' per se) in front of him. And his F-smash should have a BZZAP! sound effect.
He also needs a Mr. L costume...
(( This is where it gets really stupid ))
His tornado should spew lightning bolts all over the place. Anyone standing within close range of Luigi should receive lightning damage at the same rate or more as if they were standing off-screen. His U-tilt should drag a giant ball of electricity over his head. His FJP should also have electricity damage and constant vertical knockback (twice the height of Luigi's FJP) occupying an invisible hitbox as far out in radius from Luigi as Snake's U-tilt has horizontally. Each of his jabs should have the same hitstun as ZSS's uncharged paralyzer shots. His final smash should be his Brobot from the game Super Paper Mario coming out from the heavens and squashing everyone. His Up-B should be chargeable, such that he squashes downwards (viz-a-viz Super Paper Mario) and flies roughly as high as Sonic's Up-B at full charge.
And, and, and... Even if someone else wins a match, Luigi should still be mentioned by the announcer: "And the winner is... Luigi, just 'cuz he's more awesome than whoever you picked to fight him!"