Tanveer aka Billo Rani

When Tannu was first shown in the show she was so nice and she was in the Khan's house only to do a task for Razia. Now she has grown an obsession for Assad only after being rejected by Imraan and insulted by Razia. So how can she become such a devil in such a small lapse of time. We don't know anything about Tannu's past. Was she already such a cruel person?