The Boy Who Disappeared, also known as Boy X, was a boy that Sophie saw right before she hit her head on the sidewalk and manifested as a Telepath. He was wearing a bramble jersey, but Sophie couldn't see his face clearly, because he was wearing an addler— a gadget prevented people from focusing on the wearer. The boy had appeared out of nowhere and saw Sophie staring at him, then disappeared. Sophie started to run after him, but she tripped and hit her head on the sidewalk, causing her to go to the Emergency Room. It was then that Mr. Forkle triggered Sophie's Telepathy, and in addition, removing her memory of this incident. Sophie only got the memory back in Neverseen.
The words The Boy Who Disappeared was written in Sophie's "sparkly pink journal", and Keefe helped her recover what she'd written when she realized that the pages were missing using the pencil shading technique.
Sophie and her friends searched everywhere for The Boy Who Disappeared. They thought that he had attended Exillium and searched through the records that Dex had broken in to. They thought that The Boy Who Disappeared was the Psionipath that Sophie and Biana had encountered in the woods, Ruy Ignis.
However, towards the end, when Sophie and her friends had just escaped from the city, they are captured by Fintan and Brant. Alvar, who went with them on the mission to be their guide, joins Brant and Fintan and reveals to them to have been with the boy who disappeared all along. It turns out that Alvar was searching for Sophie. Sophie and her friends leap away just in time, telling everyone back home the news.