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, through most of the story, 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. The first time the SA-X was seen occured while Samus rushed to Sector 1 to stop the X infestation. As she went down the elevator, the SA-X blew up the wall, and then a close-up of its face could be seen. It then left, but not before destroying the door as well.

The next and real encounter occured in Sector 2, shortly after Samus had defeated Zazabi. She fell into a room with two floors, with the SA-X on the bottom floor, leaving the room.

The second encounter in Sector 6 occured when Samus was unable to pass a wall of Power Bomb Blocks, and so had to hide before the SA-X came in. The SA-X would use a Power Bomb to pass, and once it left, it was safe for Samus to follow.

The third encounter in Sector 5 happened after Samus had acquired Power Bombs of her own. As she entered a room, the SA-X suddenly entered as well. To get by, Samus had to use Bombs or a Power Bomb to find a secret passage. Unfortunately, if Samus used the latter, it also trigerred the SA-X's attention. It ran over to the wall, firing Ice Beam shots until Samus left the room.

The fourth encounter where Samus was required to get its attention occured in Sector 2 once again, shortly after Samus had restored Auxiliary Power to the station. This time, Samus had no choice but to run from the SA-X until she finally outsmarted it.

Another 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/Plasma/Wave/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 appears and transforms back into its original SA-X form. It then uses the Ice Beam against the Omega Metroid, causing it to back away from Samus. Once the Metroid reaches the end of the room, it retaliates and attacks the SA-X, draining what little vitality it had recovered after its battle with Samus. The SA-X then reverts to its X form and allows Samus to absorb it, completely restoring her energy and granting her the Ice Beam; her Fusion Suit also changes to match the color scheme of her old Power Suit. Once Samus defeats the Omega Metroid, the BSL research station crashes into SR388, and all of the remaining SA-X were supposedly destroyed along with the planet and the rest of their kin.

Why the SA-X seems to assist Samus is unclear, but it is likely that the X saw two threats: one with only a few strands of Metroid DNA (Samus), and full-blooded, incredibly powerful Metroid (the Omega Metroid). The SA-X then attacked the greater threat; if this is the case, then it probably intended to finish off Samus after the greater threat was dealt with. It is possible, however, that Nintendo was recreating the ending of Super Metroid, where the Metroid hatchling, after growing a relationship with Samus (its "adoptive mother"), sacrifices itself to save her from the final blow. The SA-X may have done the same thing.

Trivia

 * While the X was capable of infecting the bio-organic parts of the Power Suit, it was already believed that the X could not replicate robotics and technology- thus the technology and metal shell of the suit should not have been replicated. 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. The X could have just 'grown' pieces to resemble the pieces of Samus's suit; although they can use the suit's abilities, presumably through data in the biological components. Also, when Samus is fighting the SA-X it begins to look distorted as the fight goes on, which also suggests that the appearance of the suit was affected by the SA-X's will and not actually a real suit at all. Alternatively, it is possible that the suit is 100% organic or connected to Samus's genome in some way.
 * The fact that the Fedaration wanted to capture the SA-X for military use despite its obvious malicious intent is similiar to the theme of the Alien series, where the corrupt Weyland-Yutani company want to use the malicious Aliens as bio-weapons.
 * In trailers for Fusion, the SA-X's sprites appear to be directly based off of the Super Metroid Varia Suit sprite art.
 * The SA-X cannot grab onto ledges.
 * SA-X demonstrates a vulnerability to cold. This is evident when Samus fires Ice Missiles onto it in order to buy more time to escape. This mirrors Samus's occassional weakness to being frozen demonstrated in the Prime series.
 * The SA-X is one of two beings in the series that is known to have Samus' DNA, the other being Dark Samus. Other doppelgängers include Samus-G and the green training hologram in Metroid Prime Hunters: First Hunt.
 * The SA-X is one of three villains in the series able to best Samus in a fight, the others being Ridley in the Super Metroid intro and Dark Samus in Metroid Prime 3: Corruption.
 * When Samus absorbs the Core-X of the SA-X, the Ice Beam is shown on the inventory screen, but the suit she receives is not.