The servant Cook was one of Medivh's two servants at Karazhan — the other servant being Moroes. As her name/title implies, she was the tower's cook. She prepared breakfast, lunch, and dinner for the residents and guests of Medivh.
She and Moroes were both killed by their master, and were later buried by Khadgar next to Medivh's own grave outside the tower. Garona was able to find Cook's red lenses smashed when she, Khadgar, and Lothar searched Karazhan.
She was likely an old woman, wearing a set of rose-quartz lenses in order to avoid seeing the creepy manifestations and visions of magic inside the magus' tower.
She first gotten the lenses the very next day after Moroes had broken a crystal dish that Medivh had asked for; she had previously hidden the plates because she had had a vision of Moroes breaking this very one piece of crystal, several months before. She did not want to see any more visions at all.