Barbra Bellini was a prison guard aboard a prison ship headed for Justice World, carrying Simulants. The convicts escaped and there was a pitched battle with only two survivors — one prisoner and one guard, Ms. Bellini. Her fate afterwards is unknown.
The Simulant is known to have used a cryogenic escape pod and jettisoned itself into space. Thereafter the pod was found by Holly and brought aboard the Red Dwarf. The black box informed them about the battle and the survivors but they did not know whether or not the person inside was a Simulant or Barbra Bellini.
Cat activated the thaw process on the pod without knowing about the threat of the Simulant, being more interested in the woman it might contain. In response, the crew transported the pod to its original destination of Justice World and, after some issues with the Justice Computer, dealt with the Simulant there.
Bellini might not have been the only survivor, as Warden Ackerman is hinted to be the one who let the Simulants loose accidentally in the first place before coming aboard Red Dwarf. Dave Lister commented that she had a beautiful name and Cat felt for sure that she would be interested in him.