Declan is a member of The Smell.
Three nights after Charlie joined the Legends, Atropos arrived at a concert where "The Smell" was set to play. She killed Ian as well as everyone at the venue. Gilly found Declan and told him to run just before Atropos killed her, after which she interrogated Declan to find out where Charlie went. He told her she had simply vanished one day, so she killed him by revealing her true form.
Later he, Ian, and Gilly were resurrected and continued The Smell with Charlie.