A London engineer in Mortal Engines who helped develop MEDUSA. When Shrike came to Crome to help him find Hester, he allowed himself to be examined by the engineers so they could learn how to make Stalkers. Popjoy developed these new stalkers (though Shrike remarked that the London stalkers were inferior to himself and less intelligent). Later, in Infernal Devices, it is revealed that Popjoy survived MEDUSA and was taken by the Anti-Traction League and later the Green Storm. When Green Storm took over the Anti-Traction League, they were much less scared of using technology. In exchange for his life, Popjoy showed them how to make stalkers founding, the Resurrection Corps and becoming an important member of the Green Storm. After the Stalker Fangs death and resurrection, she located him to repair her, but later murdered him so he could not reveal that she was still alive.