Called "fairies" by mankind, Jack Harkness notes that these creatures do not actually have a name. Fairies are not alien life-forms, but have lived alongside humanity since the dawn of time, and although mankind has ascribed positive, friendly aspects to them, Jack insists that they are dangerous. Their exact nature is unclear, although Jack vaguely describes them as part myth, part spirit world and part reality jumbled together, mixed with "old moments and emotions", all moving backwards and forwards through time and seen only out of the corner of one's eye.

Fairies and children are linked, and Jack says that fairies were once children, taken from various time periods stretching millennia into the past. These children are the Chosen Ones, who the fairies protect and avenge if harm comes to them, until the time that they claim the children for their own. The fairies require these children in order to continue their race's existence.

The fairies are seen in two forms: one a small, glowing humanoid form with butterfly-like wings and the other a much larger, more monstrous form. They are undetectable by technology, and can appear and disappear at will. They have control of the elements, able to create sudden gales or rainstorms and direct them with pinpoint accuracy.

A common method of killing their victims is to "steal their breath", asphyxiating them by clogging their throats with rose petals. Their ability to move back and forth in time is demonstrated by the appearance of Jasmine, a Chosen One taken in the present, in fairy form in a 1917 photograph.

Jack speculates that fairies may be "part Mara".