The Siren
The “Siren” was first believed to be an enemy when she first appeared in The Curse of the Black Spot (Series 6 : Episode 3), but was later revealed to be a medical Doctor from an unnamed spaceship that had become trapped in the same space as the pirate ship, Fancy, in two separate dimensions. After the crew of the unnamed spaceship were killed by Earth bacteria, the Siren was left with no patients to care for. As a result, she became aware of the Fancy in the other dimension, and began accidently kidnapping crew members when they become injured so that she could heal them in her sick bay. If a crew member became ill or injured, such as through a simple, small cut, the Siren would take a tissue sample, leaving a black spot behind on the victim, before returning to take that victim to the sick bay.
Although the Siren would take the crew members of the Fancy to the sick bay, she had no knowledge of how to heal them, and so left them there with life support. She continued to do this even after the Doctor attempted to ward her off. During a storm, the Siren took the Captain’s son, Toby, who had a fever, and Rory, who is saved after the Doctor willingly released the Siren to save him. The Sirens song was an anaesthetic to those with the black spot on their hands and did not affect those without. Those who succumbed to the music were put in a trance like sleep and taken direct to the sick bay, whereas those who didn’t, such as the Doctor, Amy and Captain Avery, were not.
The Siren was able to appear in the form of her injured patients, but had limited intelligence. She was unable to communicate in any language or being able to detect the severity of an injury to the crew members. However, the Siren was thorough with her job, not allowing anyone else help her patients unless they could prove they were related, such as a wedding ring that Amy produced to allow her to take care of Rory, and signed a consent form. She was also very strict on germs in her sick bay, almost incinerating the Doctor for a simple sneeze. In appearance, the Siren would appear as a ghostly green figure, unless someone interfered with her work or tried to contaminate her sick bay, in which case she would become red and angry, producing fire to deal with contaminated objects, such as tissues. When Captain Avery knew that his son could not be saved, he took command of the space ship with his crew and flew off into space, allowing the Siren to care for them all.