Jasmine Pierce was a Chosen One of the Fairies.
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She was a very unhappy, lonely child who was bullied at school and at home. Her mother, Lynn, loved her dearly, but her boyfriend, Roy, disliked her and took every opportunity to be mean to her.
Unknown to them, Jasmine was one of the Chosen Ones selected by the Fairies to join them in their immortality. Until she could do so, they protected her. They killed Mark Goodson, a paedophile who tried to hurt her, and Roy, when he hit her. They also attacked the two school bullies who had made her life a misery.
Jack Harkness eventually had to sacrifce Jasmine to the Fairies to prevent others from dying. She skipped away and vanished in the glow of her Fairy friends. She was taken to the "lost lands" and turned in to a Fairy herself.
She could be seen in her Fairy form in one of the Cottingley fairy photos, frozen in mid-dance, suggesting that the photographs were actually genuine, not fake. (TW: Small Worlds)