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