Ash is the first Synthetic seen on screen in the Alien franchise and remains one of the most memorable. He is the chief Science Officer onboard the USCSS Nostromo and is played by Ian Holm. Ian Holm's likeness is also used for the synthetic, Rook, onboard the abandoned Weyland-Yutani research outpost in Alien: Romulus.
As a synthetic created to serve a specific purpose onboard the USCSS Nostromo spacecraft, there isn't much information out there about Ash's early life. What we do know, however, is he was a Hyperdyne Systems model 120-A/2 synthetic created for Weyland-Yutani and after the events of Alien: Romulus it seems clear that his specific model is used for the role of science officers about vessels that procure and study Xenomorphs.
Ian Holm as Ash in 1979's Alien
Instead of being tasked with maintaining the safety of the crew onboard the Nostromo, Ash was acting on orders to retrieve the Xenomorphspecimen at any cost. To Ash, the crew was always dispensable. His mission was merely to retrieve the Xenomorph, and he was prepared to kill to fulfill those orders.
The crew of the Nostromo also didn't know Ash was a synthetic. In this instance, there could be two reasons for the deception. The first, slighter offense is Wey-Yu wanting to test how "human" their androids can be. To see if Ash could pass as one of the regular crew, despite being a last-minute replacement. The second, far more sinister reason is because Ash had, within his programming, the mission's true priorities and if he were discovered, the Crew would know he had access to Wey-Yu reports that a regular human crew member wouldn't.
As a synthetic, Ash found the Xenomorph to be a "perfect organism," echoing David's experiments to create perfection. This could be because of synthetic's natural AI tendency to value efficiency over empathy or it could be because the Ash model was specifically designed to study the Xenomorph.
Ash did not have any reason to fear the Xenomorph that was wreaking havoc on the ship. Xenomorphs require living hosts to impregnate with a Chestburster embryo. As a lifeless synthetic, he wouldn't have been a target for them.
Ash played a pivotal role in 1979's Alien as one of the main antagonists in the film. At the beginning of the movie, Ash portrays himself as a reliable team member who is concerned with the utmost safety of his crew. Of course, this is far from the truth, but that isn't revealed until much later in the film.
The first moment where Ash's loyalties could be questioned is when he lets executive officer Thomas Kane back on board the ship, despite the fact that he was already compromised by a Xenomorph Facehugger. This went against the team's already firmly established quarantine rules, which warrant officer Ellen Ripley was trying to enforce
Ripley confronts Ash in 1979's Alien
Ash also refuses to freeze Kane, instead preferring to perform experiments on the specimen and ensure the safety of the Xenomorph instead of containing the potentially life-threatening species.
Ellen Ripley, the main protagonist of the first four Alien movies, questions Ash's allegiances from the moment he breaks the quarantine rules. After she uncovers his true orders to save the alien specimens for Weyland-Yutani by any means necessary, he attacks her to prevent her from revealing his plans to the rest of the crew. This is when Ripley, and the audience, finds out Ash is a synthetic.
Ash, destroyed, in 1979's Alien
Thankfully, Ash is intercepted by other crew members while he's trying to suffocate Ripley by shoving a rolled-up magazine down her throat. After his demolished body answers some of Ripley's questions, it's set on fire by Nostromo engineer Dennis Parker.
The 'Ash' in Alien: Romulus is an interesting case. By the time the film was released in 2024, Ian Holm had passed away and was unable to reprise his role as a synthetic onboard the Weyland-Yutani ship. Instead, the producers of Alien: Romulus were able to digitally recreate his likeness via CGI, prompting an interesting debate about the morality of hijacking people's identities after they have passed away.
Rook in Alien: Romulus
While the synthetic in Alien: Romulus does look like Ash, he is not 'Ash' per se. He is the same model synthetic as Ash, which explains the similarities, but his name is Rook, and he is indeed a different character. In addition to their appearance, however, Ash and Rook also share being Wey-Yu science officers in charge of studying the Xenomorph. In this case, the same actual Xenomorph since it's the same one -- Xenomorph XX121 -- in both movies.
Rook, however, gets pretty far with his research about The Renaissance, reverse engineering Black Goo from his various xeno-work. After his research ultimately leads to the Xenomorph breaking containment and the entire staff being killed, or used as hosts, Rook sees an opportunity to salvage the Black Goo when mining colony thieves break into the outpost and their synthetic, Andy, gets rebooted to follow Wey-Yu orders. The plan fails and Rook is destroyed when the Renaissance crashes into the rings of LV-410.