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Derek Beresford is the son of Tommy and Tuppence Beresford. He is the twin brother of Deborah.

In the novel N or M?, Derek is in the Air Force. He is described as having a "wide grinning smile" which is very like Tommy's. He writes a letter to Tuppence, telling her not to worry about him, and that he would not have "missed this show for the world". He also mentions two of his friends, Gus and Trundles, who want to be remembered to Tuppence.

Derek and his sister, Deborah, affectionately refer to Tommy as Carrot Top.

At the end of the novel, Derek returns home on leave, and dances at a party hosted by Tommy and Tuppence. Deborah mentions that he is going to be recommended for something, but he stops her before she can say what it is. He then tells his parents that it involves "some show all of us are doing", and looks "as embarrassed as though he had been accused of the most deadly of sins".

In the novel By the Pricking of My Thumbs, Derek does not appear, but Deborah does mention him during a telephone conversation with her father, when she says that none of them ever think of Tuppence as Prudence.

Tuppence also mentions Derek during a conversation with Miss Bligh in Sutton Chancellor. She tells Miss Bligh that he is married, and has children.