One of Yoshi's friends

Luigi is portrayed as the taller, younger brother of Mario, and he is usually seen dressed in green with overalls. Although Luigi is a plumber, like his brother, other facets of his personality vary from game to game. Luigi always seems nervous and timid but is good-natured and not as quick to anger as his more famous twin.
While it has not been made official, Daisy may be Luigi's romantic interest. They were a romantic couple in the film  and in Mario Kart Wii they are seen in statue dancing together.  She was his caddy in NES Open Tournament Golf as Peach was to Mario. Also on Daisy's trophy in Super Smash Bros. Melee, it says that she is possibly Luigi's answer to Mario's Peach. So Luigi may not be as popular but without him, Yoshi and Mario would definitely have more problems.