Like other Plasmavores, she subsisted on blood for nourishment. She fled to Earth to escape capture by authorities, and had the ability to change her internal physiology to match that of native life forms if she had also absorbed their blood.
Checking into Royal Hope Hospital, Florence claimed to be overcome with dizziness. Julia Swales suggested that they do a CT scan or some blood tests, but Dr. Stoker said that she was simply salt-deficient as she had only eaten salad over the previous days. It was actually the case that, being a Plasmavore, she absorbed salt very well. She commanded two Slabs, one of which was killed by the Tenth Doctor and the other by the Judoon. She also owned a stripy red-and-white straw with which to suck her victims' blood.
Her victims included the Child Princess of Padrivole Regency 9 (whom she killed on a whim) and Dr B. Stoker, a doctor at the Royal Hope Hospital, whom she killed to assimilate his blood so she would appear biologically human to the Judoon scanners. She also drained the Doctor of blood, and in so doing nearly killed him. However, this enabled the Judoon to recognise her as not human, and therefore the fugitive that they had to capture/execute. The Judoon ultimately decided to kill her with their pistols, but not before she activated the MRI. (TV: Smith and Jones)
The Plasmavore calling herself "Florence Finnegan" showed an externally pleasant, friendly and harmless manner. In reality, the Plasmavore was a cruel, murderous and extremely sadistic individual. She expressed an intense bloodlust to Mr. Stoker, to the point that he showed utter disgust at what she was saying, and she had absolutely no qualms about drinking other people's blood. She showed a cunning and highly intelligent nature, assimilating blood of humans to disguise herself from the Judoon and concocting a plan to slaughter everyone in the Royal Hope Hospital so she could commandeer a Judoon ship to escape alone.
The Plasmavore seemed not to care a single iota for the thousands of people on the Earth that would die as a result of this, stating that the side of the Earth not facing the Moon's survival was 'her little gift'. She showed an impulse towards killing, declaring contemptuously that the Child Princess of Padrivole Regency 9 was "begging for the bite of a Plasmavore". She expressed utter pride in her actions when confronted by the Judoon, and then taunted the Judoon to enjoy their victory because they would 'burn with her', gleefully accepting her execution with the self-assurance that they would die with her. (TV: Smith and Jones)