SA-X
Metroid character
SA-X in Metroid Fusion
First appearance	Metroid Fusion (2002)
Designed by	Yoshio Sakamoto Katsuya Yamano

The SA-X is the main antagonist of the video game Metroid Fusion. She is a parasite that originally infected the protagonist, Samus Aran, as well as her Power Suit, before Samus was cured by injecting Metroid DNA into her. The SA-X later appears, having replicated her Power Suit, including all of her most powerful weapons from Super Metroid. She is pursuing Samus throughout the game, who is much weaker and thus must avoid her until she is able to fight back against the SA-X.

The SA-X was designed to mimic Samus' movements and actions, with designer Yoshio Sakamoto identifying such a concept as unsettling. The concept of being chased by the SA-X was reused in the video game Metroid Dread with the antagonists, the EMMI, as Sakamoto wanted to reproduce that tension.

The SA-X has been met with generally positive reception, praised as an iconic character as well as one of the scariest characters in video games. Her power, particularly in contrast with Samus', was met with praise, with critics identifying the chase scenes between the SA-X and Samus as the best moments in Metroid Fusion. She has also been compared to the EMMI in Metroid Dread, which were themselves inspired by the SA-X.

When designing Metroid Fusion, designer Yoshio Sakamoto aimed to make the SA-X mimic Samus' movements and action, feeling that it was an unpleasant concept to have someone mimic someone. Enemy programmer Katsuya Yamano identified SA-X as the thing he spent most of his time on.^[1][2][3] The SA-X was also designed to have all of Samus' equipment from Super Metroid, contrasting a defenseless Samus.^[4] The video game Metroid Dread features robots called EMMI that hunt down Samus. The concept comes from SA-X, with Sakamoto wanting to recreate the tension SA-X produced in another game.^[5] They specifically wanted to feature the chase style of gameplay in a Metroid game with more traditional gameplay than Fusion.^[6]

The SA-X originally appeared in Metroid Fusion, having developed from an X parasite originally found on the planet SR388. It first infects both the protagonist Samus Aran and her bio-organic Power Suit after Samus killed the creature it was originally inhabiting during a Galactic Federation research mission on the planet. Samus nearly dies but is saved by having Metroid DNA injected into her body, with most of her Power Suit having to be surgically removed. This Metroid DNA ultimately makes her immune to infection, due to Metroids being natural predators of the parasite.^[7]

The X parasite residing in her Power Suit remains eventually transforms into a replica of Samus on the BSL Space Station, able to use the arsenal of weapons and upgrades she wielded in Super Metroid, while Samus herself is left relatively weakened and unable to survive the SA-X's ice weapons.^[7][4] The SA-X stalks Samus throughout the game, forcing her to hide or flee for most of it. As the plot develops, the player discovers that there are multiple SA-X, the X virus having replicated itself, and are targeting the station's Metroid breeding facilities, which includes Samus due to her DNA. When Samus learns of the Galactic Federation's intention to capture the SA-X, potentially allowing the parasite to spread across the galaxy, she programs the station to crash into and destroy SR388 to wipe out the X.^[8] After Samus has grown powerful enough, she faces off against the SA-X, managing to kill it even after it evolves into a more physically imposing form.

After being defeated, the SA-X flees in its parasite form, reforming later when Samus is facing off against an Omega Metroid. It is ultimately defeated by the Omega Metroid, at which point Samus fuses with it and regains her powers from Super Metroid, allowing her to kill the Omega Metroid and escape from the space station before it collided into SR388, destroying both.^[9]

The SA-X has been generally well-received by critics, considered one of the best new villains of 2002 by Nintendo Power.^[10] Retro Gamer staff called her iconic, attributing her icon status to her overwhelming power and intimidation.^[11] Retro Gamer writer Darran Jones also noted her first appearance, where she looks straight at the player with her "cold, dead eyes", which they stated still makes their stomach "turn in terror".^[12] She has been identified as a particularly frightening encounter in video games, with GamesRadar+ writer Connor Sheridan finding her the "coolest part" of Metroid Fusion.^[13] Hardcore Gaming 101 writer Kurt Kalata considered the moments with the SA-X to be the best part of Metroid Fusion, though felt that the boss battle was too basic.^[7] GameSpot writer Jordan Ramée commented on SA-X being "[Samus] at her best", and how it reflected how Samus, now infused with Metroid DNA, was being hunted by herself, reflecting how she once hunted the Metroids to extinction. He praised the writers for creating a character so "chillingly horrifying" that they are still memorable years later.^[14]

Dual Shockers writer Marcus Jones considered her the scariest Nintendo character, citing her original appearance - where she blows her way through a door and slowly walks - as a "dreadful" moment. That Samus has to evade the SA-X rather than fight her was considered by Jones to be Nintendo's "greatest unorthodox take on survival horror".^[8] VG247 writer Fran Ruiz compared SA-X to the titular antagonists of The Thing and The Terminator, as well as Nemesis T-Type from Resident Evil 3: Nemesis, all of whom stalk their respective protagonists. She felt that running and hiding from the SA-X was stressful, but that the final encounter feels "earned and empowering", saying that she is her "personal boogeyman to this day".^[15] Yahoo! News staff identified her as the most frightening stalker character in video games, feeling that she is the "real meat of the horror" in Fusion.^[16] Nintendo World Report writer James Dawson considered her the standout antagonist in Metroid, saying that encounters with her were among his "most memorable moments in gaming" from his childhood. He noted that it was the first time he felt helpless in a video game.^[17]

The SA-X has been compared to other characters in the Metroid series, including the EMMI from Metroid Dread, with the EMMI criticized as being an inconvenience compared to SA-X.^[18] Jordan Ramée felt that the EMMI was an evolution of the SA-X concept mechanically, but a step back thematically. He felt that revealing that the EMMI could be killed early in the game took away from its impact, noting that the SA-X being a "terrifying threat" is enhanced by being invulnerable to Samus until the end of the game.^[14] Kurt Kalata felt similarly, feeling that EMMI are an improvement mechanically, and while they created "some incredibly tense standoffs", the SA-X were more frightening.^[19] Nintendo Life writer Ollie Reynolds felt that Dark Samus lacked impact due to SA-X, feeling SA-X was superior and Dark Samus felt "been there done that".^[20]