Cassidy is the 21-year-old sister of Posey. She's described as being tall and skinny, with hair like Posey's and a freckle under one eye. Cassidy is a yoga teacher and taught the Girl Guides how to do yoga during Christmas break.
The Girl Guides track Cassidy down to the Higher Ground Yoga Retreat. As the Guides plus Runner Five reach the compound, it appears that Cassidy is going to be cannibalized. However, it turns out that Cassidy has stolen and hidden all the compound's guns, and the other hippies are simply trying to realign her chakras with tuning forks.
The hippies state that Cassidy is the best hot yoga teacher in London, or possibly the world.