Mason is not imaginary. He's not a ghost, either. And he's most definitely not a hallucination.
Mason is an Aerling, and the Sentinels' number one target. Inseparable since Olivia found Mason crying in her front yard as a child, being apart from each other is unbearable. Knowing the separation is necessary to keep Mason safe and alive should make it easier for Olivia, but being stripped of her best friend and the person she loves most only heightens her fear and suspicions of those claiming to help. The pain of being kept apart until Mason's eighteenth birthday, when Olivia is expected to guide him back to the world of the Aerlings, is the least of their worries, though. As the Sentinels intensify their search for Mason, they bring the threat of danger to a level no one is prepared to face.