The SA-X is the main antagonist in Metroid Fusion. When Samus Aran was attacked by the X Parasites on SR388, her Power Suit was contaminated. To save her life, the Galactic Federation medical teams had to remove her contaminated suit and fuse her with Metroid DNA in order to absorb the X. Her Power Suit was moved to the BSL Research Station, where the X Parasites that remained on the suit reanimated it and commenced reproducing and spreading the parasite to other life-forms on the station. This animated suit became the main focus of Samus Aran on the mission, and was dubbed Samus Aran X, or simply the SA-X. The SA-X is, like most parasites, able to split itself and reproduce asexually. When the SA-X learns of Metroid presence on the station, its priority becomes to destroy them at all costs, as Metroids are natural predators of the X, and were bred by the Chozo to stem their reproduction.
The SA-X is armed similarly to a fully powered version of Samus Aran's Power Suit, utilizing Morph Ball, Power Bomb , Screw Attack and Ice Beam weapon systems. Since Samus was rendered almost completely powerless as a result of having her suit stripped from her during the medical procedure to save her life, she had no choice but to flee from the SA-X on a number of occasions. The Ice Beam in particular was incredibly dangerous to her, as the injection of the vaccine made from Metroid DNA also gave her the Metroid's weakness to being frozen. Samus finally faces the SA-X in battle as she heads to the Operations Room in order to destroy the station.
The SA-X is immune to most of Samus's weaponry, and only charged beam shots will damage it. Diffusion Missiles can be used to stun the SA-X, and Screw Attacking the SA-X will cause both of you to take damage. Using the overhangs in the area, it is possible to avoid the SA-X's attacks, and before long it will phase in and out of existence and drop to its knees. When Samus approaches the fallen SA-X, it will mutate into its true form, a mutated version of the host creature that infected Samus in the first place. It will hop around the room and try to stomp on Samus. Again, only charged beam shots will hurt it, and doing enough damage will destroy the creature, revealing its Core X. When Samus defeats the Core X, however, the X Parasite inside flies away.
The SA-X finally meets its end when it appears after Samus has been brought to her knees by the Omega Metroid. The X's fear of the Metroids causes it to attempt to kill the Omega, but it is no match for the highly evolved Metroid and is defeated, allowing Samus to absorb its Core, granting her the Ice Beam, which she uses to destroy the Omega Metroid before escaping the crashing BSL Station in her Gunship.