Fairy

Fairies (as the Human race called them) were dangerous Humanoid entities native to the Earth. Fairies were not alien life-forms, but had lived alongside Humanity since the Dawn of Time. Their exact nature was unclear, although Jack Harkness described them as part myth, part spirit world and part reality jumbled together, mixed with "old moments and emotions".

Powers and Abilities
Fairies had a wide variety of advanced, seemingly magical abilities. They had control over the elements, and were able to control the weather to produce localized, but deadly, storms and gales. Not bound by normal physical laws, they could choose whether or not a camera could take their pictures. For example, they made themselves invisible to closed circuit television cameras on one occasion. On another, they enabled themselves to have their photographs taken. They could travel through time and had an apparent non-linear relationship to it as demonstrated by a picture of Jasmine Pierce, who became a Fairy in 2007, in her fairy form in 1917. A common method of killing their victims was to "steal their breath", asphyxiating them by clogging their throats with rose petals.

Fairies could be seen in two forms; a small, glowing Humanoid form with butterfly-like wings and a Human-sized, more monstrous form. Both forms had wings and could fly with ease. At least in the larger form, Jack Harkness described them as only visible from the corner of one's eyes. In their larger form, they spoke with voices that were both soft and errie, making it hard to tell wether they were male or female. (TW: Small Worlds)

Interactions with Humans
Jack Harkness told Gwen Cooper that Fairies came from the Dawn of Time. Humans, over time, made Fairies into something harmless and beautiful, however those who had encountered them, like Jack, never doubted their powers.
 * Jack speculates that fairies may be "part Mara."

According to Jack, Fairies were once Human children, taken from various time periods stretching millennia into the past. These children were called the Chosen Ones, who the Fairies protected and avenged if harm came to them. Eventually, the Fairies claimed the children and took them to their "Lost Lands". Once there, the children were doomed to become Fairies themselves. The Fairies required these children to continue the existence of their race. These "Chosen Ones" always seemed slightly odd and would try to spend as much time as possible with the Fairies.

The Fairies were not generally involved with the affairs of other Humans, but they violently defended any infringements on their territory and killed anyone who they felt had acted against them. (TW: Small Worlds)

They could also distinguish between those who harmed the Chosen child and who didn't, as when Jack was on a train with a group of soldiers who had run over a Chosen One, the fairies attacked the men, but left Jack unharmed in a car full of dead bodies once they were done. The Fairies also appeared to have limitless powers over the elements, as Jasmine Pierce, a Chosen One, told the team when they tried to stop her from being taken they would "turn the world to ice". When Jack said that would mean no more Chosen Ones, Jasmine revealed the Fairies would simply keep to the past.