Yes, Lexie was Cassie's imaginary friend after the car crash. Then, when Cassie required an 'undercover' identity, she used the name Lexie. Then - with me so far :-)) - a real life Lexie doppelganger turns up dead. So now, Cassie is assuming the identity of the dead Lexie to infiltrate the 'gang of four' who all live in the same house. I presume she is doing so to try and discover who killed Lexie. Or, to be exact, who 'thought' they had killed Lexie. This series is based on the first 2 books of a series and are therefore intertwined, so i'm assuming that this particular aspect is from one while the child killings are from another.