Ariana McCall was Scott's aunt and his dad's younger sister. She has always been there for Scott and Stiles through their whole childhood. For a 27 year old she is pretty strong. Stiles used to have the biggest crush on her since he was 7 and she was 17. Then as she got older he gave up.
When Kate ( Ariana's ex-bestfriend) terrorizes the town, the group turns to Ariana for help. She used to know Kate really well and there were some important things that she shared with her in the past that she has forgotten. The only option left is to make her think that she's a teenager again. But in the process something goes wrong and she turns out to be more of a teen than she thought.
Season 4 of Teen Wolf.