Mewtwo

Mewtwo is a character that debuted in Melee, was planned for Brawl but was scrapped, and will return in 3DS / Wii U as DLC.

In Super Smash Bros.
Mewtwo was planned for Super Smash Bros. 64, as stated on the Japanese website

In Super Smash Bros. Melee
Mewtwo is an unlockable character in Super Smash Bros. Melee

Moveset

 * Neutral: Shadow Ball
 * Side: Confusion
 * Up: Teleport
 * Down: Disable

How to Unlock

 * Play 20 hours of VS Mode
 * Play 700 VS games

In Super Smash Bros. Brawl
Mewtwo was planned to be in Brawl, and Lucario is thought to replace her.

In Super Smash Bros. 4
Mewtwo will be a downloadable character in SSB for 3DS / Wii U.

Moveset

 * Neutral: Shadow Ball
 * Side: Confusion
 * Up: Teleport
 * Down: Disable
 * Final Smash: Psystrike

How to Unlock

 * Register the 3DS and Wii U version on Club Nintendo by 3/31/2015 to get him for free on April 15, 2015.
 * Purchase him for $4.99 on April 28, 2015.