The SA-X is a 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 the infected parts and sent the pieces to the Biologic Space Laboratories research station's Quarantine Bay. Once there, the infected suit parts transformed into a X resembling Samus. The SA-X was able to use a Power Bomb to escape the Quarantine Bay, which caused the capsules containing all the X to break open, starting the infestation.
From then on, the SA-X roamed the station, crossing paths with Samus and attacking her on sight. With the use of the Ice Beam, Samus stood no chance against the SA-X due to the Metroid vaccine. Thus, whenever Samus encountered the SA-X, she would have to hide or run from it, which happened on more than three occasions. One of these occasions showed the SA-X fighting Metroids in the Restricted Area, though they overpower it. Apparently, the SA-X had released the Infant Metroids, causing the Restricted Area to be locked down, and eventually caused it to break away from the research station and explode, with Samus barely escaping.
After the incident, Adam told Samus that there was were no fewer than 10 SA-Xs and that they asexually reproduce. The Galactic Federation took an interest in the SA-X and believed that it had strong potential. The computer then goes on to tell Samus that the Galactic Federation did not expect Samus's help, which is why they stopped sending her support data.
Once the computer updated to Adam's personality, he told Samus to go the Operations Room and adjust the station's orbit path to intercept SR388. Once Samus reached the Operations Room, she encountered an SA-X and finally battled it.
Samus at first seems to best the SA-X, but then it transforms into a gigantic beast, a sort of mutant hybrid Hornoad-like creature, which was the host of the X before it infected Samus in the first place. Once this form is defeated, it will transform into a Hard Core-X. However, once Samus defeats the Hard Core-X, the X inside floats away from Samus before she can absorb it.
Once Samus reaches the Docking Bay where her gunship should be, she instead encounters an Omega Metroid. The Omega Metroid attacks Samus, leaving her with an ounce of her life left. Just as the Omega Metroid is about to deal the finishing blow, the X that floated away from Samus after defeating the SA-X appeared and transformed back into its original SA-X form. It then used the Ice Beam against the Metroid, causing it to back away from Samus. Once the Metroid reached the end of the room, it retaliated and attacked the SA-X, causing it to bounce back in front of Samus. It then transformed back into an X, allowing Samus to absorbb it. She then gained the Ice Beam and the Omega Fusion Suit. Why the SA-X seems to assist Samus is unknown, but it is likeliy that the X saw two threats; one that has only mere strands of Metroid DNA (Samus), and the other being a incredibly powerful Metroid (the Omega Metroid). It then attacked the greater threat.
Once Samus defeated the Omega Metroid, the BSL research station crashed into SR388, and all the SA-X were supposedly destroyed.
In addition to the common abilities of the X, the SA-X demonstrates the following abilities of Samus' Power Suit:
Despite having Samus's power and a high intellect, according to Adam, the SA-X is infamous for its incredibly low artificial intelligence. In one example, the SA-X Power Bombs a floor above a Morph Ball tunnel and reveals Samus if she isn't quick enough, but if Samus goes into the non-destroyed part rather than firing at SA-X, it will Screw Attack to the top part and back down to the bottom in an endless cycle. In another example of its lack of intelligence, if Samus stands on top of one of the various pillars she uses to hide from the SA-X, her opponent, seemingly not being able to comprehend the fact that it needs to jump higher to reach Samus, will try to Screw Attack up to her but will always fall short, and it will keep trying the exact same method until Samus leaves its sight.