Initially appearing to be a harmless history teacher at Amberwood Prep, it was soon revealed at the end of Bloodlines that she is actually a witch of sorts: A human who dabbles in magic and the supernatural and is part of a coven of witches. Terwilliger is perfectly aware of the world of Moroi, Strigoi, Dhampirs and Alchemists. It was revealed that she had known all along that Sydney was an alchemist and saw a magical talent within her, one that she wanted to nurture. Because of Sydney's alchemist beliefs however (who strongly detest anything supernatural or otherworldly) Terwillger resorted to using a passive aggressive method of teaching Sydney. She has largely been successful in this endeavour, driving Sydney to create charms and cast spells by herself, as much as she despises it. Nonetheless, these magical practices have proven useful to Sydney.
she continues to try and convince Sydney to train her abilities (again using the passive aggressive method) and even gives her a more modern magic textbook that contains spells for practical defense. When Sonya is kidnapped by the Warriors of Light, Sydney comes to Ms. Terwilliger's house for help, and performs the scrying spell in order to locate Sonya.
- Ms. Terwilliger owns thirteen cats.
- She is also the first practicing witch to be introduced in the series.
- Richelle Mead originally planned for Ms. Terwilliger to be a man in order to give Sydney a better father figure, but eventually scrapped the idea.