Deacon Maybury was a Sanctuary Sensitive who specialized in a controversial process called re-identification, in which criminal personalities were overwritten and replaced with more peaceful ones. He was a former member of Foe's gang, who plotted to destroy the world.
Deacon Maybury used to be a member of Foe's gang until he betrayed them, stealing the key to the Doomsday Machine in hopes of selling it. The gang then killed his identical brother, Daffyd Maybury, with a wood-chipper. To hide from them, he disguised himself as a 15 year old boy called Ryan with the help of Robert Crasis who created the new life of Ryan with Maybury encased in his subconscious. Ryan was then kidnapped by Foe's gang, but was saved by Skulduggery Pleasant and Valkyrie Cain. As Maybury still resided in Ryan's mind, he forced Ryan to subconsciously try to stop Skulduggery from reaching the Shopping Centre where they would dismantle the bomb.
After the Doomsday Machine was de-activated, he returned to his old self while Valkyrie kissed him. He was allowed to flee as Ryan was a friend of their's, not before Valkyrie punched him in the face.
He was briefly seen and helped in the re-identification of Argeddion, creating a new personality for him. Skulduggery explained that after they did him a favour in letting him escape, he was repaying them.