Cancer was the name of an infamous assassin employed by Servalan to kill the crew of the Scorpio. Cancer's appearance, age, and planet of origin were all a mystery to the galaxy at large, and was wrongly believed to be a man.
Cancer became renowned as an infallible killer, maintaining her anonymity throughout.
Cancer and Servalan (Cancer's 'utiliser') concocted a complex plan to allow the assassin to infiltrate the Scorpio crew. This began by deliberately letting them intercept a message arranging a meeting between the two on the planet Domo. Believing that Cancer would abandon the contract if Servalan was dead, the Scorpio crew travelled to Domo in an attempt to eliminate her before Cancer received the mission.
Servalan arrived on Domo shortly before Cancer and purchased an actor from one of the slave auctions there. The actor was taken to Cancer's ship when it arrived. Cancer persuaded the actor to pretend to be the famous assassin, promising they would 'go off together' once the mission was complete.
The Scorpio crew, having missed Cancer on Domo, caught up with her ship. Avon and Tarrant boarded the vessel and fell for Cancer's deception, believing the actor to be the killer they were after. Cancer herself adopted the identity of Piri, a highly-strung slave girl. Her plan was to get the crew to take her back to their base where she could pick them off one by one.
Cancer killed the impersonator when he became a nuisance, and Nebrox when it appeared he might realise she was not who she claimed to be. She also planned to kill Avon and Soolin, who were not as accepting of her disguise as Tarrant, but failed in both cases.
She set her crab on Soolin before knocking Avon unconscious. Pausing only to change her hairstyle and makeup, she revealed her true identity to him. However, Soolin was not as dead as Cancer believed and appeared to rescue Avon. Cancer's venomous crab was flicked onto her by Soolin, and she was poisoned and died by her own creation.
For her final mission Cancer used a black ship marked with the sign of the crab. All the systems of this ship were remotely controlled from Servalan's vessel, so it may have been provided for her by Servalan.
Cancer killed using what appeared to be a venomous crab which injected a deadly toxin. Through unexplained means, when it was not being used to kill the crab transformed into a rather gaudy metal brooch with the crab symbol on it. The crab may have been robotic but splattered spectacularly when shot by Del Tarrant.
While masquerading as Piri, Cancer portrayed a spectacularly wet and irritating young girl with a serious inferiority complex, presumably to secure her place in Tarrant's affections.
As herself Cancer was highly contemptuous of her victims, ruthless and without remorse.