Ripley (Sigourney Weaver) was the badass hero of Alien (1979). More importantly, she rewrote the precedent for women characters in sci-fi and horror moving forward.

She demonstrates an appreciation for rules and regulations. She doesn’t let emotion overwhelm her sense of duty. She’s intelligent and insightful. She figures out the truth about the alien’s presence on the Nostromo. She doesn’t let Ash choke her to death with a porn magazine. As the rest of the screw starts to drop, she takes charge. And for all that, she is the only one who survives the film. Even when the film teases us with its fourth act, where Ripley "escapes" only to find the alien followed her to the shuttle, she blasts it out of the airlock and into space.

Early reports of the Alien script indicate that Ripley was written as a man, but Ridley Scott cast a woman and defied the conventions of the time. Sci-Fi and horror prior to 1979 were largely campy B-movies with predictable plots and male-driven hero characters. Casting Ripley as a woman not only broke that mold, but expanded the potential audience and fan base of the two genres that didn’t traditionally appeal to women as a whole. It helped establish the film as what is now considered a classic piece of cinema and lent credibility to unreconstructed genres.

Ripley paved the way for leading women in the field of sci-fi, from Sarah Connor (Linda Hamilton) in The Terminator to Trinity (Carrie-Anne Moss) in The Matrix, to sci-fi television heroines like Buffy (Sarah Michelle Geller), Alpha and River Tam (Summer Glau) in Joss Whedon’s female-lead series Buffy, Dollhouse and Firefly.

From The Guardian:

“As played by Weaver (then 29 and a relative unknown), [Ripley] initially seems set up to play the conventional role of the sexy scream queen. But after a while, we realise that not only is she not screaming, she also appears to have no romantic interest whatsoever. Instead, she furrows her brow and tackles tasks with a steely determination. It is Ripley who makes the right decision in refusing to allow the alien aboard the ship, only to be overruled by her male colleagues. For good measure, it is Ripley whose misplaced maternal instincts (hurrying off in search of the ship's cat) enable her to avoid the monster's attack.

Only at the end does the film let her down, slapping us with that oddly gratuitous sequence in which Ripley escapes in a space-pod and proceeds to strip down to her undies. It is as though the makers were so alarmed by what they had unleashed that they tried to rein her back at the last minute. "No one would deny that Sigourney Weaver looks fetching in her underwear," chuckles David Thomson. "But that's really a case of Ridley Scott playing to a very old-fashioned gallery."

Whether or not Ripley’s underwear scene served to undermine the advancement of female protagonism the movie sought so hard to construct is a separate, debatable matter entirely, with decades of cinematic argument.  But no matter what side of that argument one falls on, it’s safe to say that in the over 35 years since Alien was released, Ripley being female made an impact on the genre.