Lucas

Lucas is the main protagonist of Mother 3, a Japan-only video game. Lucas has debuted in Super Smash Bros. Brawl as a playable character and has appeared in the Smash series since his first appearance in Brawl. He has been a playable character in the Smash series since his debut in Brawl.

Super Smash Bros. Melee
When Sakurai updated Ness's profile page on July 17, 2001, he explained that Lucas was supposed to replace Ness as a playable character in Super Smash Bros. Melee. However, it was decided that Ness would be retrieved as a playable character instead due to the cancellation of Mother 3's original iteration on the Nintendo 64.

To quote Sakurai on the matter:

Both Ness and Lucas would later appear in Super Smash Bros. Brawl as playable characters.

Super Smash Bros. Brawl
Lucas makes his Smash debut (as well as his worldwide debut) in Brawl, joining Ness as a playable character. Lucas is a starter character, unlike Ness.

According to Masahiro Sakurai, he stated that when he found out that Mother 3 would not be released internationally, he decided to forget about his plan of replacing Ness with Lucas, and both characters would be playable to represent the Mother series.

Super Smash Bros. for Nintendo 3DS / Wii U
While he was absent upon the initial release of SSB4, Lucas appears in the game as a DLC fighter. His moveset is the same and has been buffed and nerfed a little bit.

Super Smash Bros. Ultimate
Lucas returns along with everyone else in Ultimate. He is an unlockable character by adventuring through World of Light, playing 350 minutes of VS. Mode and having 35 characters unlocked, or clearing Classic Mode seven times as Kirby or anyone in his unlock tree.

Trivia

 * Had Mother 3 not been delayed, Lucas would have appeared in Melee as a playable newcomer, and he was going to replace Ness completely.
 * Lucas is the only known character in the Smash series to appear in just one game outside of Smash, with that being the long-awaited Japan-exclusive game Mother 3.
 * If Lucas had been used in Melee as a playable fighter, he would have made a total of 5 appearances in the series instead of 4 appearances, but this plan did not make the cut during the development of Melee.