The SA-X (short for Samus Aran-X) is the main antagonist of Metroid Fusion.
The SA-X is an X Parasite mimicking Samus Aran. After Samus's first encounter with the X on SR388, her Power Suit was infected by the parasites. The Galactic Federation surgically removed infected portions of her suit and sent the pieces to the Biologic Space Laboratories research station's Quarantine Bay. Once there, the X inside the infected suit parts seemingly combined and transformed into a copy of Samus in her standard Varia Suit.
