Dark Samus

Dark Samus is the main antagonist of the Metroid Prime subseries of the Metroid series debuting in Metroid Prime 2: Echoes. She is a doppelganger of Samus Aran and a form of Metroid Prime, created after it merged with Samus's Phazon Suit to survive following its defeat on Tallon IV in the original Metroid Prime.

Super Smash Bros. Brawl
In Super Smash Bros. Brawl, Dark Samus does not appear in a physical form, though she appears as both a collectible trophy and a sticker, both depicting her appearance in Metroid Prime 2: Echoes. In Subspace Emissary, her sticker can only be equipped by Samus and Zero Suit Samus, and gives them a +11 increase in their battering resistance when equipped.

A "Dark Samus" appears in co-op Event 7: Battle of the Dark Sides, though while it is meant to represent this character, this Dark Samus is merely Samus's model recolored to be completely black.

Super Smash Bros. for Nintendo 3DS / Wii U
Dark Samus appears as an Assist Trophy in Super Smash Bros. for Nintendo 3DS and Super Smash Bros. for Wii U, taking her appearance from Metroid Prime 3: Corruption. She attacks using a variety of Phazon-based abilities, such as tentacles that sprout from the ground, a burst of shots from her arm cannon, or spherical missiles that home in on enemies. If enough damage is dealt to her, a ball of light forms around her before she disappears; otherwise, she simply disappears after a period of time. Dark Samus is also the basis for one of Samus's alternate colorations.

Super Smash Bros. Ultimate
Dark Samus debuts as an unlockable playable character in Super Smash Bros. Ultimate, being an Echo Fighter of Samus and sharing the same moveset but with different properties. She also floats above the ground in a fashion similar to Mewtwo. Dark Samus's Final Smash is similar to Samus's Zero Laser, though it is colored black rather than blue.

Trophies
Blue indicates exclusive to the Wii U version.