SA-X

The SA-X (short for Samus Aran-X) was the main antagonist of Metroid Fusion and the eleventh boss Samus faces on the Biologic Space Laboratories research station. It was an X Parasite mimicking Samus Aran.

Physical appearance
Owing to it having copied Samus's Varia Suit, and to a lesser extent DNA from Samus herself, the SA-X is a near-splitting image of Samus Aran herself. The only noticeable difference between the SA-X and the actual Samus is that the SA-X has blank eyes behind the visor.

Origin
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 released from a Hornoad-X she had killed. The Galactic Federation surgically removed infected portions of her suit and sent the pieces to the Biologic Space Laboratories research station's Quarantine Bay. Once there, the X inside the infected suit parts seemingly combined and transformed into a copy of Samus in her standard Varia Suit (it should be noted in SA-X's first closeup its eyes appear to be white, with no visible pupils). The SA-X used a Power Bomb to escape the Quarantine Bay, breaching the capsules containing X-infected specimens from SR388, and started the mass infestation.

Encounters with Samus
From then on, the SA-X roamed the station, occasionally crossing paths with Samus and attacking her on sight. As it had the use of the Ice Beam, Samus originally stood no chance against it due to the Metroid vaccine leaving her with the Metroids' only weakness: an extreme vulnerability to cold. Thus, whenever Samus encountered the SA-X, she would have to hide or run from it to progress.

The copy was first seen when Samus was traveling to Sector 1 to try and stop the X infestation from spreading. As Samus descended in the Main Elevator, the SA-X blew up a wall of the elevator room, walked into the room, and paused for a few moments; it is here that a close-up of its face could be seen. The SA-X then jumped over the elevator shaft, destroyed the room's hatch with a Super Missile, and left the room through the destroyed hatchway. When Samus reaches Sector 1, the computer informs her that an "unknown agent" had severely damaged the Main Elevator and disabled it for the time being.

Upon Samus' arrival in Sector 2, the computer briefed her about the SA-X, informing her that it disabled the Main Elevator, told her about its abilities and to avoid it at all costs. The second encounter occurred shortly afterward, which was after Samus downloaded the Bomb data. As soon as Samus left the Data Room, an explosion shook the surrounding area, immediately after which the SA-X's footsteps could be heard fading away into the distance. When Samus dropped down to the lower part of the room, she discovered that the hatch she had come through had been badly damaged and rendered totally inoperable.

Samus' first physical encounter with the SA-X also occurred in Sector 2, shortly after the defeat of Zazabi. Attempting to return to the Navigation Room, Samus discovered another damaged and completely impassable hatchway. Using her Bombs, she was able to open a hole in the floor nearby and dropped into a room with two sections. The SA-X was in the bottom section, but it soon left the room. After leaving Sector 2, Samus wondered whether the SA-X was actually a mindless creature or not.

The next encounter, this time in Sector 6, occurred when Samus discovered a wall of Power Bomb Blocks that she could not pass, as she had not yet recovered her Power Bombs. The SA-X entered the room almost immediately afterward, forcing Samus to hide in a passageway under the floor; after walking to the wall, it then used a Power Bomb of its own to destroy both the wall and a significant portion of the floor. It then jumped down the next area and exited the room.

The third actual encounter took place in Sector 5 after Samus regained her Power Bombs through downloadable data. While using Power Bombs to get around blockages created by the X, Samus entered a portion of the sector she had not previously explored. Almost immediately afterward, the SA-X suddenly entered the room right behind her. To escape, Samus had to jump over a pillar and use Bombs or a Power Bomb to find a secret passage in the wall behind it. Unfortunately, if Samus set off a Power Bomb, the explosion would destroy the pillar behind her, revealing her position to the SA-X and alerting it to her presence; it would then run over to the wall and fire Ice Beam and Wave Beam shots until Samus left the room. The SA-X can also be alerted when a charged shot is fired in its vicinity. If a normal Bomb is used, the SA-X will not detect Samus.

The SA-X showed up once again in Sector 2 shortly after Samus activated the Auxiliary Power Station. While traveling towards the source of the vegetation choking the Central Reactor Core, Samus dropped into a large chamber only to discover that the SA-X was there waiting for her. As she still lacked a way to damage it, Samus was left with no choice but to run from the SA-X until she could reach a safe hiding place, though she could slow it down by hitting it with Ice Missiles, which would freeze it for the time being.

When Samus' navigation computer requested that Samus destroy the B.O.X. Security Robot, he warned Samus that the SA-X was hunting her in Sector 6 and ordered her to leave the sector immediately after defeating B.O.X. But before destroying B.O.X., she found the entrance to the Restricted Laboratory. After defeating the robot, with her newly regained Wave Beam, Samus entered the Lab and discovered the Galactic Federation's secret Metroid breeding project. Following the sound of explosions rocking the lab, she found the SA-X attacking Infant Metroids, though it was quickly overpowered and overwhelmed. However, the SA-X's assault had activated the emergency fail-safes, meaning it would detach and self-destruct. Moments after Samus escaped, the Restricted Area was jettisoned from the research station and exploded. Samus believed that the X mimic was gone for good.

However, her feelings of victory were short-lived. After the incident, Samus' navigation computer told her that there were no fewer than 10 SA-X because the X reproduce asexually. Shortly after, the Galactic Federation had taken an interest in the SA-X, believing it to have strong potential as a bioweapon The computer went on to tell Samus that the Federation did not expect her help in containing the SA-X, which is why they had stopped sending her support data., and tried to detain Samus in the room until she accidentally called the computer "Adam".

Once the computer was revealed to have Commander Adam Malkovich's personality, he ordered Samus to go to the Operations Room and adjust the station's orbit path to intercept SR388, effectively wiping out the X Parasites for good. Just as Samus reached the Operations Room, an SA-X returned and ambushed her in an attempt to stop her from destroying the station.

By this time, Samus had recovered almost all of her lost abilities; even without the Federation's help, she had recovered the Gravity Suit and Plasma Beam, giving her enough defense and power to almost match the SA-X. Therefore, she was able to finally confront her doppelgänger, and in the fight that followed the SA-X was found to be vulnerable to the Wave Beam's power, its form destabilizing as it took damage. In response, it mutated into a huge, misshapen monster in a desperate effort to crush Samus, yet she emerged victorious. As with most major X hosts, it re-assumed its base Core-X form firing the Ice Beam from its shell, yet was again defeated. However, unlike other Core-X, it escaped to parts unknown before Samus could absorb it. Samus then begins the interception sequence and rushes to her Gunship. However, when she reaches the Docking Bay where the Gunship should be, she instead encounters an Omega Metroid. The Omega Metroid attacks Samus, reducing her energy to one unit. Just as the Omega Metroid is about to deal the finishing blow, the SA-X that fled after Samus defeated it reappears and transforms back into its Varia Suit form. It then uses its 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 both the Ice Beam and an Unnamed Suit. Once Samus defeats the Omega Metroid, her gunship returns, allowing her to escape. The BSL station then crashes into SR388. All of the remaining SA-X were supposedly destroyed along with the planet and the rest of their kind.

The reason why the SA-X seems to assist Samus is unclear in-game. However, the game's instruction manual reveals the only clue regarding this behavior, stating that the X Parasites react adversely to the presence of nearby Metroid tissue. Therefore, it is likely that it saw two threats: Samus, an individual with little Metroid DNA, and the full-blooded, incredibly powerful Omega Metroid, prompting the SA-X to instinctively attack the greater, natural predator. If this is the case, then it probably intended to finish Samus off if it emerged victorious against the Omega. It is worthy to note that this final battle pays homage to that of the previously released game Super Metroid, in which the baby, whom initially attacked Samus in an earlier encounter, sacrifices itself to save her from Mother Brain.

The battle
The SA-X primarily uses the Ice Beam and Screw Attack during the battle. However, it will always turn to the use of Super Missiles whenever Samus hangs from a ledge, firing them at a random surface of the room and taking advantage of the weapon's tremor effect in order to make her lose her grip. The SA-X can, on very rare occasions, fire its Super Missile at the exact ledge where Samus is found, causing massive damage to her.

Only the Charge Beam can damage the SA-X, while Ice Missiles can only temporarely immobilize it. The SA-X has a tendency to go into a Screw Attack as a counter-attack whenever Samus lands a blow; if Samus tries somersaulting into it with her own Screw Attack, both will take damage. As the SA-X takes more damage, its physical shape becomes increasingly distorted, with pieces of organic matter falling off of it; once it cannot take any more damage, it falls to its knees (like Samus herself at the end of Super Metroid, and again at the end of Fusion).

When the SA-X kneels down, it stops fighting. Moments later, it transforms into a gigantic beast that appears to be a sort of mutated Hornoad-like creature with the colors of the Varia Suit. A possible explanation for this form is that it was a fusion of every host the SA-X had previously infected, namely a Hornoad and Samus herself. Noticeable details on this form are the various Arm Cannons on one of its arms and the suit's visor having changed into a large one above its mouth with multiple, smaller visors on the forehead. Despite having Arm Cannons, the SA-X jumps around the room trying to stomp on Samus. Ultimately, this tactic proves much less effective than when the SA-X was only mimicking Samus, as there are ways to exploit its jumping pattern in order to avoid damage: if Samus stands on the upper ledge in front of the Operations Room door, she can use the Wave Beam to shoot down through the floor, but the SA-X can't jump high enough to reach her. Alternatively, Samus can use the Screw Attack to somersault right through the SA-X; this won't hurt the SA-X, but Samus won't take any damage, either. In order to damage the SA-X, Samus must once again use the Charge Beam.

Once the mutated form is defeated, the SA-X reverts to a Core-X and attempts to both collide onto Samus and hit her with the Ice Beam. Once Samus defeats the Core-X, the X inside retreats out of reach, phasing through a wall and escaping.

During the course of the game, the SA-X demonstrates use of Samus' Super Missiles, Ice, Plasma, Wave and Spazer Beams, Morph Ball, Power Bomb, Space Jump and Screw Attack.

Artificial intelligence
Despite having Samus's powers and, according to Adam, a high intellect, the SA-X is infamous among fans for its incredibly poor artificial intelligence coding. There are various holes and ledges that Samus can hide on or in that will cause SA-X to endlessly run and jump back and forth without hitting Samus, sometimes even if she is within easy reach.

One such example occurs in the first encounter of Sector 2: If Samus attracts attention, it will chase after her, but if she climbs up the hole she came through, the SA-X will run to the wall, lose interest in the chase and walk calmly across the room to leave.

In another example, in Sector 6, the SA-X Power Bombs a floor above a Morph Ball tunnel, revealing Samus if she isn't quick enough; if Samus rolls into the intact part of the tunnel on the right side (the part where she came in through) and is seen doing so, it will Screw Attack and Space Jump to the top part and back down to the bottom in an endless cycle as long as she stays there.

Also in the same scene, if Samus falls into the Fake Block chamber and hangs there while the SA-X is alerted, it will drop down to the lowest part of the room and constantly run back and forth across the room.

Additionally, in the next room, when Samus climbs into the Morph Ball shaft, the SA-X, like before, will constantly run back and forth.

When hiding from the SA-X in Sector 5, if Samus fires any projectile or drops a Bomb behind the pillar (without destroying it) when the doppelganger turns to leave, the SA-X will turn around as if it had heard something and walk back to the pillar, but it does not extensively search the room or even shoot its Wave Beam at the pillar and will shortly lose its interest before leaving unless its attention is attracted again. This goes on until Samus stops or she reveals herself to her opponent. This also occurs in the second encounter of Sector 2, at the very last room, although it stays near the door.

In the second encounter of Sector 2, if Samus is to crouch in the one-square-wide space after the third shutter in the room before the aforementioned pillar, the SA-X will jump over Samus from atop the pillar to the ground and back with no end until Samus leaves the area. The SA-X even continues this regardless of what Samus is actually doing in the space, whether she is in Ball form, crouching, standing, shooting beams or missiles in any direction, or even Bomb Jumping (up to how tall Samus would be standing in the space) to taunt the SA-X. If Samus jumps in the same encounter, the SA-X turns around and heads to the edge of the slope. If a Power Bomb is dropped, it shoots at where it thinks Samus is, repeatedly.

Notably in Sectors 6, 5, and the second encounter of 2, if Samus stands on top of one of the various pillars she uses to hide from SA-X or goes into a narrow Morph Ball tunnel but stays close to the edge her opponent, unable to comprehend the fact that it needs to jump higher to reach Samus or aim and fire at her, will try to Screw Attack up to her but will always fall short, and it will keep trying the same method until Samus leaves its sight.

The SA-X can act like this without knowing Samus is there if she places a Power Bomb in the right spot during the Sector 5 encounter, The bomb will destroy the pillar but not alert the SA-X (indicated by SA-X Appears not playing). Regardless, the SA-X will still try to enter the tunnel by Screw Attacking.

In addition, when entering the Restricted Laboratory, despite clearly having access to the Ice Beam and Super Missiles, the SA-X simply fires erratically at the Metroids as the area was being locked down and jettisoned, eventually being latched on by various Metroids. However, it may have been trying to destroy the laboratory outright to quickly neutralize its natural predators even at the cost of its own life, like the Scientist was doing with the entire station.

Despite this, canonically the SA-X is a truly intelligent being. For instance, the SA-X destroyed the Quarantine Bay with a Power Bomb in order to escape. It also destroyed numerous Data Room locations and/or as much access to them as possible in an attempt to prevent Samus from regaining her abilities. Additionally, the SA-X predicted her movements: it was waiting for her in Sector 2 as it knew she was going to try to destroy Nettori as well as following her into Sector 6, the Restricted Laboratory and the Operations Deck. If Samus hung on a ledge out of the SA-X's reach, it will fire a Super Missile in order to dislodge her due to the rumble created by said Missile. Finally, it knew the most efficient way to destroy a Metroid; freeze it with the Ice Beam, then shatter it with a Super Missile. This is because the X are able to absorb knowledge from their victims, in this case the SA-X has Samus' knowledge of how to eliminate Metroids.

Official website
"While exploring SR388, Samus was infected by an X Parasite. Samus survived the infection, but so did the parasite. Now an exact replica of Samus at full power, SA-X may be the most potent foe Samus has ever faced."

Trivia

 * In trailers for Fusion, the SA-X's sprites appear to be directly based on Super Metroid ' s Varia Suit sprites.
 * Despite this the SA-X's sprite is asymmetrical. When facing the right its shoulder armor appears to be relatively flat, when facing left its shoulder armor more closely resembles the traditional large shoulders of the Varia suit. Though this is not the case with images of the SA-X in cutscenes.
 * Chronologically speaking, SA-X is the second doppelganger/clone of Samus Aran, with the first being Dark Samus. While created through different means, both are shown to be powerful adversaries who tested Samus' abilities. However Dark Samus was shown to have more personality, while SA-X was simply an X mimicking Samus at her full power.
 * Interestingly, both Dark Samus and SA-X granted Samus a beam upgrade of some kind at some point. Samus gained the ability to use the Hyper Beam after Dark Samus infected her with Phazon (albeit through the PED Suit the Federation gave Samus) and the Ice Beam (in addition to the Unnamed Suit) after Samus absorbed SA-X.
 * Ironically, the SA-X belonged to a species which had been preyed upon by the species Dark Samus originated from.
 * The SA-X can be temporarily frozen (though only for a very brief moment) if Samus fires Ice Missiles at it, buying her time to escape. Interestingly, Metroid Fusion is the only 2D game in the series to allow Samus herself to be frozen as well, while the Prime Series has many instances where this is possible.
 * The SA-X takes double damage when frozen, though it requires excellent timing at a specific distance to have Samus' combined beam weapons inflict maximum damage.
 * Curiously, the SA-X's Ice Beam lacks the ice particles and the wavy effect seen on Samus's.
 * Samus' new Screw Attack resembles something akin to a buzzsaw after absorbing the ability from Neo-Ridley, while the SA-X uses the green Screw Attack present in Super Metroid. Additionally, the Screw Attack sprites may depict the SA-X as if it were the Gravity Suit, as evidenced by the purple hue.
 * The SA-X is referred to using gender non-specific pronouns, though it could be considered female since the X Parasite used Samus' genes to form the imitation (as was the case with Dark Samus). It is, however, referred to as a male in one instance in Metroid Prime and Metroid Fusion: Prima's Official Strategy Guide.
 * By using the Debug Menu to turn off the Wave Beam during the final fight with the SA-X, it can be seen that the SA-X cannot be hurt by the Plasma Beam alone, and that the Wave Beam is necessary to damage it with a charge shot.
 * The Ice Beam also cannot hurt the SA-X, even when charged.
 * Despite Adam stating there were no fewer than ten SA-X in existence, none are encountered if she explores the station later in the game.
 * During the final stage of the SA-X fight, if Samus deals the finishing blow to the Hornoad-like creature at a great distance (far enough to make the boss barely visible onscreen), it will stand in one place, convulse and emit the same painful screech repeatedly until Samus approaches it. Upon doing so, it will slouch and transform into a Core-X.
 * The SA-X is one of the four bosses who necessarily engages Samus more than twice over the course in their respective games, the others being Dark Samus in Metroid Prime 2: Echoes, Ridley in Metroid Prime 3: Corruption, and the Rhedogian in Metroid: Other M.
 * If the Hornoad-like creature is defeated at a specific frame, a glitch may occur where it becomes invincible instead of transforming into a Core-X, the only way to finish the game will be to reset and load a previous save.
 * The SA-X can only chase Samus into certain rooms; if Samus enters a room where the SA-X can't follow her, it disappears. This is most notably displayed in the first Sector 2 encounter: if Samus survives exiting the room after being spotted, the SA-X doesn't follow, and is mysteriously absent when reentering.
 * SA-X Appears, the theme that plays when the SA-X is nearby and chasing Samus, was composed by Minako Hamano.
 * Although not appearing in any other games, it was given an indirect reference in the Adventure Mode for Super Smash Bros. Brawl, Adventure Mode: The Subspace Emissary, where the Sector 1 theme plays while Zero-Suit Samus and Pikachu fight off against two Samus clones.