SA-X

SA-X (also known as Samus Aran-X) is the tenth boss Samus faces aboard the Biologic Space Laboratories research station in Metroid Fusion. Although it is not chronologically the final boss in the game, it is the game's primary antagonist and main adversary.

Biography
The SA-X is an X Parasite mimicking Samus. After Samus's first encounter with the X on SR388, her Power Suit was infected by the parasites. The Galactic Federation removed the suit surgically 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(it should be noted in SA-X's first closeup,the helmet and upper torso seem to be a shade of purple). 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. It had the use of the Ice Beam, meaning that Samus stood no chance against the SA-X, due to the fact that the Metroid vaccine left her with an extreme vulnerability to cold. 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 were no fewer than 10 SA-X and that they asexually reproduce, or reproduce without a mate. 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.

Artificial Intelligence
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, if Samus stands on top of one of the various pillars she uses to hide from 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. When hiding from the SA-X after the powercut in the station, if Samus fires behind the pillar when the doppelganger turns to leave attracts its attention again, but it does not extensively search the room and will leave after it loses interest. During the course of the game, the SA-X is seen demonstrating the following abilities of Samus's Power Suit:
 * Super Missile
 * Ice Beam/Spazer
 * Screw Attack
 * Morph Ball
 * Power Bomb
 * Space Jump

The Battle
The SA-X uses the Ice Beam, Screw Attack and sometimes Missiles (it fires them when Samus hangs from a ledge out of reach to shake her down) against Samus. Only the Charge Beam can damage it; Samus can temporarily freeze it with an Ice Missile, though it causes no actual damage. The SA-X has a tendency to go into a Screw Attack to counterattack whenever Samus lands a blow; if Samus tries somersaulting into it with her own Screw Attack, both she and the SA-X will take damage. As the SA-X takes on more damage, it starts to look distorted, and once it cannot take any more damage, it kneels on the floor.

When it kneels down, it stops fighting. When Samus goes near, though, it transforms into a gigantic beast, a sort of mutant, hybrid Hornoad-like creature; the host of the X before it infected Samus in the first place was a hornoad. One possibility about this form is that it was a fusion of every host the SA-X had possessed, which would also explain what looks like a swollen Arm Cannon on one of its arms. The SA-X then jumps around the room and tries to stomp on Samus. There is a safe spot that can be used during this stage of the fight: if Samus stands on the upper ledge in front of the Operations Room door, she can shoot down through the floor, but the SA-X can't jump high enough to reach her. Again, only using the Charge Beam can damage it. Once this form is defeated, it transformed into a Hard Core-X. However, once Samus defeats the Hard Core-X, the X inside retreats out of reach, eventually phasing through a wall and escaping.

The Conclusion
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 infuse it. She then regained the Ice Beam and was wearing the new suit.

Why the SA-X seems to assist Samus is unknown, but it is likely 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, or it could be that is was Nintendo's way of recreating the ending of Super Metroid, where the Metroid hatchling, after growing a relationship with Samus (Adoptive mother in this case), saved her from the final blow, healed her and giving her the Hyper Beam, the SA-X does the same thing.

Once Samus defeated the Omega Metroid, the BSL research station crashed into SR388, and all of the remaining SA-X were supposedly destroyed.

Trivia

 * It is interesting to note that, while the X was capable of infecting the bio-organic parts of the Power Suit, it is already known that the X cannot infect robotics and technology- thus the technology and metal shell of the suit should not have been infected. Yet it is known that the SA-X had multiplied at least 10 times in the course of Fusion, thus raising the question as to how the SA-X copies obtained the technological aspects of Samus's suit.