The year was 1983, and Luigi got his first appearance in none other than the arcade game, Mario Bros.. In this game, Luigi was simply a green palette swap of Mario, in order to give the game another character, to be controlled by a second player.
His first appearance as a character that wasn't just a palette swap of Mario was in the Japanese Super Mario Bros. 2, where Luigi could jump higher and farther than Mario, but controlled with less precision; he would slide around a little more. These traits have continued throughout the series, but it was the European and American release of Super Mario Bros. 2 that Luigi was truly identifiable as being a different character.
The American and European Super Mario Bros. 2 was simply Doki Doki Panic, but with altered graphics to fit the Mario series. In this game, Luigi was given his trademark height, meaning he was no longer a Mario palette swap. These combined features are what makes Luigi what he is today, literally!
Since then, Luigi's had his fair share of adventures, mostly as a sidekick to Mario in some way. For example, he appears in Paper Mario, but stays at home whilst Mario goes on to save Princess Peach. This changed in Luigi's first starring role, Mario is Missing, although the game was mainly educational and wasn't a great period in Luigi's life. Luigi's character was developed during this time; he was shown to be cowardly and lived in his brother's shadow for the most part.
After that, Luigi played a part in the successful Mario & Luigi series. Jokes are often made at Luigi's expense, but all in all, Mario couldn't have saved the world without his brother's help. These jokes help build Luigi's character further, developing him into the loveable hero we know today.
Later in the Mario series, Luigi ventures to the Waffle Kingdom to save Princess Eclair, a tale you can hear about in Paper Mario: The Thousand Year Door. But then in Super Paper Mario, Luigi becomes a playable character and a villain when he becomes the brainwashed Mr. L! You'll have to play the game to find out what happens in the end! Luigi then saves the day and Princess Peach in Super Mario Galaxy... but only after Mario does it first.
Of course Luigi's played his fair share of sports too, featuring in countless Mario sports titles, including Golf, Tennis and even Baseball. Luigi has also been a playable character in every Mario Party and Mario Kart title to date, and that's unlikely to change.
So here we are at the present day, and it's the year of Luigi. Luigi's popularity can only grow, especially with the release of his third game, Luigi's Mansion 2 coming up in March. The newest entry in the Mario & Luigi series features gameplay inside Luigi's dream themselves and Mario Golf: World Tour will feature Luigi as a playable character once again.
Looking back, it's clear to see that even though he started out as a simple palette swap, Luigi's done very well for himself and gained much appreciation over the years. It's clear Luigi has become one of the biggest characters in gaming and while it's unlikely he'll be more popular than Mario, as long as he carries on being the Luigi we all know and love, that's fine with us.