ONE? ...Are you saying there is more than one SA-X?
— Samus Aran
The SA-X, presumably an abbreviation for Samus Aran-X, are a group of X that mimicked Samus Aran. This particular strain of X parasites was created by the infected sections of Samus's Power Suit when she was on an escort mission for Biologic Space Labs researchers on SR388. The first SA-X awakens in the Quarantine Bay and escapes using a Power Bomb, and begins wreaking havoc throughout the B.S.L research station. Though it was originally believed that there was only one powerful SA-X, like all X, SA-X can reproduce asexually and produced several copies of itself. As a result, there were at least 10 SA-X on board the station towards the end of events on the B.S.L station.
Encountering the SA-X
Throughout the events of Metroid Fusion, Samus is instructed constantly to avoid any SA-X encountered, since they are stronger than Samus in her weakened state. The SA-X's most powerful weapon against Samus is the Ice Beam due to the crippling effect of the Vaccine "Metroid". Samus stood little chance of victory, let alone survival, against an SA-X until she obtained the Varia Suit to protect her from the Ice Beam and powerful weapons that can return heavy fire (namely the Ice Missile and Wave Beam).
The First SA-X
The SA-X is originally detected by B.S.L's systems when it Super Missiles its way through a wall in the Main Deck. Samus's AI navigator gathered data that implied that this SA-X had set off a Power Bomb in the Quarantine Bay that released itself and the X that were originally released on the Research Station. The SA-X continued to Super Missile various doors in Sector 2 (TRO) forcing Samus to find ways around them using her newly recovered Morph Ball Bomb. Samus encounters it in this Sector briefly, where it displays its Ice Beam to open a simple Blue Door. Samus then encounters it while she is in a vent as a Morph Ball. This time, the SA-X uses its fearsome Power Bomb to overcome a barrier.
The SA-X continues to complicate events by hacking into Sector 3 (PYR)'s heating systems. This incredible display signifies that the X seems to gain the cognitive ability of the creatures' whose DNA it absorbs, if it didn't already have it. Samus then encounters it in Sector 5 (ARC), where she has now gained many of her abilities to protect herself against the powerful foe with, but not yet the ability to defeat it. Samus and the X in Sector 2 (TRO) again, shortly prior to her battle against Nettori. The SA-X chases Samus until it loses sight of her.
An SA-X is seen destroying barriers around Metroids and attempting to fend them off and failing in the restricted areas of Sector 6 (NOC). Its destruction triggers the ignition of fires, creating a hazard reaction to detach the restricted area from the rest of the ship for self-destruction.
As Samus returns to the Main Deck, an SA-X (probably the first SA-X) challenges her to a final match. Without the option of fleeing, Samus fights the creature armed with the Wave Beam. Even with these weapons, Samus at first only manages to damage the SA-X to the point where it transforms to a grotesque, mutated form. Once this creature is injured, the SA-X reverts to a blue Core-X, but escapes before Samus can break its protective shell. This Core-X is later absorbed by Samus after an Omega Metroid devastates its armor. By absorbing the SA-X, Samus regained her abilities and upgrades at her peak strength, including her original Varia Suit and the Ice Beam, which she uses to defeat the powerful Omega Metroid.
- SA-X despite its supposed intelligence, is often infamous among fans for its poor AI coding during its encounters:
- Out of many examples, it can be "trapped" in several locations by having Samus stay in particular locations (such as hiding in specific corners with the Morph Ball). The SA-X will then continue to repeat an action such as somersaulting repeatedly with the Screw Attack, trying to hit Samus to no avail.
- There are times where Samus can easily take rest on a high ledge inside certain rooms during specific encounters (Sectors 6, 5 and the second encounter at Sector 2), and the SA-X will always try to hit Samus physically, instead of jumping a bit higher than needed and occasionally trying to shoot her if she's further away.
- Depending on the distance Samus is close towards the SA-X, it may always use a normal standing Ice Beam shot; an example is that during the encounter at Sector 5, if Samus attracts its attention with a Power Bomb and hides near the opening of the tunnel in Morph Ball form, the SA-X will never aim upward when close enough despite the height difference.
- When hiding from it, the SA-X may also have its attention attracted when Samus does something to do so; most of the time the SA-X will not bother to search around extensively and will peacefully walk on. Doing so repeatedly will always cause the SA-X to keep looking around, but will not cause it to make a move unless Samus reveals herself.
- Thus, the SA-X will normally stop repeating its actions when Samus leaves its sight.
- The SA-X was named by Adam.
- "I have named it the SA-X. I believe the SA-X came from the capsule containing your infected suit parts. It used a Power Bomb to escape the Quarantine Bay. That explosion breached the capsules containing all the X that started this disaster. But the SA-X is definitely our biggest worry right now. The SA-X is mimicking you at full power. You can't face it. If you see the SA-X, just run. Don't think about fighting... You are still very vulnerable to cold, an unexpected side-effect of the Metroid vaccine. Like Metroids, you can be frozen by an Ice Beam. Of course, the SA-X is armed with the Ice Beam. Stay away. If you see it, just run. HQ says they have Bomb data ready in the Data Room. Bombs will help you find a way out if the SA-X traps you. The Data Room is here, but... it's sealed by a Level 1 hatch. You'll have to find the Security Room and release the Level 1 locks. But per safety protocols, it's not marked on the map. You'll have to find it on your own somehow, Samus. So, first find the Security Room and unlock Level 1 hatches, then go to the Data Room and get that Bomb data." —Adam Malkovich (Metroid Fusion)