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John Marx was Olivia Marx's older brother. He was a slight rival of Sam Roth because he had a deep crush on Grace Brisbane. He was deeply saddened when his sister died and got into a physical fight with Sam because he blamed him for Grace's and Olivia's disappearances.
Grace describes him as attractive, but is later surprised by how she could have possibly thought so when she and Sam meet John and his sister, Olivia at the Crooked Shelf.