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 without a doubt the game's primary antagonist and main adversary.

Biography
The SA-X is a 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. 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 singularly. 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 of its lack of intelligence, 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.

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 and Screw Attack against Samus. Only the Charge Beam will damage it, but you can freeze it with an Ice Missile, though it causes no actual damage. The SA-X has a tendency to go into a Screw Attack when it gets hit; if you somersault into it with your own Screw Attack, you both will take damage. As the SA-X takes more damage, it starts to look distorted and once it can not take any more damage, kneels on the floor. If a Gameshark, Action Replay, or other hacking device is used to obtain the Ice Beam, it does not affect the SA-X. There is a very simple way of defeating the SA-X. If standing on the bottom level just under the overhang they shoot the SA-X with a charge beam the second that it jumps down it will usually jump upwards again. If it does not use a ice missle to freeze it and run past.

Once Samus approaches the SA-X, 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. One posibility about this form is that it is a fusion of every host the SA-X had possesed, which would explain what looks like a swolllen arm cannon on one of its arms. It will jump around the room and try to stomp on Samus (there is a safe spot during this stage of the fight; if Samus is on the 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 will damage it. Once this form has been defeated, it will transform into a Hard Core-X. However, once Samus defeats the Hard Core-X, the X inside floats away from Samus.

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 a 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, it grows somewhat of a relationship with Samus, saved her from a final blow, healed her, and gave her a new suit, and the Ice Beam.

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