When, at the end of Alien, Sigourney Weaver says, “This is Ripley, last survivor of the Nostromo, signing off,” she’s not telling the whole truth. Because there’s another survivor, curled up with her in the hypersleep capsule. Jones (or Jonesey) the ginger tom.
Jones serves multiple functions within the Alien storyline:
1) CATGUFFIN, a pretext for characters to go wandering off on their own.
2) CATPANION, an excuse for Ripley to express herself out loud when she’s otherwise alone.
3) CATSHOCK, a cheap shock tactic in which the cat jumps out unexpectedly.
4) CATSCALLION, a wild card; at the end of the film, the cat might yet be harbouring an alien.
In short, one cannot overestimate the importance of Jones to Alien. This is his story.