Jennifer Troke had the ability to vanish, like Barry Blake. She retained her powers at the end of the fourth book.
She was described as a willowy blonde with glasses.
She was better at vanishing than Barry, and was known (amongst the COLAs) for the ability of reappearing at your elbow, commenting on your book. She often got Barry into trouble during lessons, but was not in trouble herself that much, because she was able to look innocent. Barry was often seen in her company, suggesting their friendship.
Dax Jones commented she "looks like butter wouldn't melt in her mouth."
In the fifth book (Stirring the Storm),Gideon Reader tells Dax that she was attracted to Dax, because she appeared to stare at him, and blushed when he came nearby. Once, she even faded out when he was nearby.