Lucas is the main protagonist of Mother 3. The son of Flint and Hinawa, he is characterized as more timid than his twin brother, Claus. While playable briefly during the prologue of Mother 3, he is an NPC for the first three chapters of the game, with Flint, Duster, and Salsa being the main characters of those chapters. After reappearing in the climax of Chapter 3 against the Pork Tank, he becomes the main playable character after the three-year time skip to Chapter 4, and retains this role for the remainder of the game.
While he initially leaves his hometown of Tazmily Village to locate Duster, he quickly becomes involved in a greater struggle against the Pigmask Army. He learns PSI, and his ability to use PK Love sets him into a chase against the Masked Man to pull four of the Seven Needles to awaken the Dark Dragon living under the Nowhere Islands. Lucas's main party of allies includes Kumatora, Duster, and Boney.
In battle, Lucas fills the same archetype as his predecessor Ness did in EarthBound, being somewhat slow, but having a strong basic attack and access to a variety of PSI techniques such as stat boosts, Lifeup, PK Flash, and the aforementioned PK Love. Where previous Mother protagonists wielded baseball bats, Lucas generally wields sticks.
In Super Smash Bros. Melee
When Masahiro Sakurai updated the original Smabura-Ken with Ness's profile on July 17th, 2001, he said that Lucas was meant to replace Ness as a playable character, but Ness was included due to time constraints, and likely due to the cancellation in 1999 of EarthBound 64, the first version of Mother 3 that had been worked on.
Both Ness and Lucas would ultimately appear as playable characters in Super Smash Bros. Brawl.
In Super Smash Bros. Brawl
Main article: Lucas (SSBB)
Lucas makes his Super Smash Bros. series debut, and by extension his international debut, in Brawl. His special moves are similar to Ness's, but with some key differences. For example, Lucas's PK Thunder goes through opponents and can deal multiple hits, while Ness's PK Thunder disappears on contact. Lucas's Final Smash is PK Starstorm, which he shares with Ness. Lucas also has near-identical physics to Ness's. The majority of Lucas's moves completely differ from Ness's, leaving him as a semi-clone of Ness.
Lucas is ranked 30th on the tier list in E tier. His low-tier placing is primarily due to the additional 10 frames he gets when grab released (a trait he shares with Ness), which makes him vulnerable to grab release follow-ups from every character, including zero-death chain grabs from Marth and Donkey Kong.
Main article: Lucas (SSB4)
Although he was not present in the final roster upon the initial worldwide release, Lucas was released as a DLC character on June 14th, 2015. His non-fighter trophy appeared in both versions of the game, in addition to his special moves which are available as custom special moves for Ness. 
Lucas currently ranks at 28th place out of 55 characters on the Smash 4 tier list in C tier (tying with Ness). As with Ness, Lucas no longer possesses the extra grab release frames, a main contributor to his low tier position in Brawl. His Rope Snake tether recovery has also been made more reliable, thanks to Smash 4's new edge mechanics preventing edge-hogging. However, Lucas received various nerfs to his damage output, knockback and range. He also lost his useful Magnet Pull and Zap Jump techniques. However, update 1.1.3 presented a significant upgrade to Lucas's grab game, as now his tether grab has noticeably less cooldown, making it reasonably fast for a grab of its nature. Because of the buff, Lucas can more reliably set up combos with his throws, most noteworthy being down throw, which can easily link into at least two neutral aerials at low percentages. Lucas is overall considerably more viable than he was in Brawl, and he has gained a dedicated playerbase and decent results in competitive play.
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