|For other characters named Derek, see Derek.|
Derek was part of the Responders' rapid response team, acting as a guard and escort. He was responsible for making contact with the residents of Harpers Ferry after the bombs dropped and, after seeing their need for assistance, offered to send a representative from the Responders who could stay in Harpers Ferry more permanently.
After Harpers Ferry was attacked by scorchbeasts, Derek was disappointed to learn that Raleigh Clay and the other members of the Free States intended to retreat to their bunker and no longer offer help to the Responders. In his final report regarding the situation at Harpers Ferry, he asked Maria Chavez to contact Ella Ames to ask her to speak to Raleigh and try to convince him to change his mind.
Derek Castle is mentioned only in Fallout 76.