Ripley was not the focal point of Alien until the last couple reels though.
Well one way or another she was the film’s lead.
SilverWook said:
She was just part of the Nostromo crew. Her sticking to biohazard protocols by not allowing Kane back into the ship, (before Ash circumvented her) even made her seem like an unsympathetic character at first.
That concept is called interesting character (character development). Something that is missing in TFA.