Although Luigi is typically overshadowed by his older brother, Mario, Luigi is unique all his own. He stars in the Luigi's Mansion games, where he takes on hoards of ghosts in haunted houses. His cowardly personality doesn't typically aid him in those situations.
In Smash Bros., Luigi plays both very similarly and very differently to his red-capped brother. The two share many attacks, but they have substantial differences, making them feel completely unique. While Mario excels on the ground, Luigi rules the air with his fast and powerful aerials. He is quite floaty in the air and has very little traction while running. Luigi also has a new Final Smash - he pulls out the Poltergust 5000, sucking up all his opponents before sending them flying out into the sky.
Text by Professor Squeaky, Taojaz, & Smashedpotatoes