Jack Ferguson was the older son of Robert Ferguson, Sherlock Holmes' client in "The Adventure of the Sussex Vampire". He was fifteen at the time of the case. Jack is described as an odd boy, unusually attached to his father and bearing a character of "strong likes and dislikes". He had an extremely antagonistic relationship with Mr Ferguson's Peruvian second wife, as well as towards his infant half-brother. Holmes confirms this when he observes the boy's reflection as others, particularly his father, are looking after the baby. Ferguson sought Holmes' help after discovering that his had what seemed to be vampiric tendencies towards her infant son. Holmes realises that Mrs Ferguson had in fact been trying to save the baby's life, as Jack, furious with rage, had attempted to murder him twice times using a Peruvian blow-dart his father kept as a trophy in the house: Mrs Ferguson had been sucking the poison out from his wounds. Holmes recommends a surprisingly lenient punishment for Jack: a year at sea.

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