Katie Russell was Owen Harper's fiancée before he joined Torchwood 3. She was thought to suffer from Alzheimer's disease, which would make her the youngest case ever. Owen did not trust this and ordered examination after examination. None showed up anything unusual. After a last brain scan the doctors thought she had a brain tumour. During the operation it turned out that she was infected by an alien creature, which in self defence killed the doctors and Katie herself, devastating Dr. Harper.
She was working as a junior doctor at the same hospital as Owen. (TW Fragments)
