Imogen was a member of the female Martense coven. She, along with Filla and Symbaline were sent to Lorwich by the coven's queen to vanquish the evil there.
The three split up, with Imogen exploring the Lorwich mine, while Symbaline and Filla each explored the church and trainyard, respectively.
None of three sisters returned, which prompted Celeste to head to Lorwich in order to find out what happened to them, and to rescue them if they were still alive. Celeste managed to find Imogen first, but was already too late to save her.