John Trelawney is a wealthy landowner and a key figure in the expedition to find Captain Flint's treasure, serving as the primary financier of the entire voyage. He commissions the schooner Hispaniola and is heavily involved in recruiting its crew, though his judgment in this area proves to be dangerously flawed. A bombastic and excitable individual, Trelawney possesses a childlike enthusiasm for pirate lore and the romance of treasure hunting, often displaying an infectious energy that drives the plot forward from its earliest stages. He is described as tall and broad, with a roughened, reddened face from his prior travels and prominent black eyebrows that hint at a quick temper.

Trelawney's most defining personality trait is his profound gullibility, which is paired with an inability to keep sensitive information confidential. Despite explicit warnings from his friend, Dr. Livesey, to maintain secrecy about the treasure map and their destination, Trelawney's loose lips lead him to inadvertently reveal critical details about the voyage to nearly everyone in port. This lack of discretion, combined with his naive and overconfident nature, allows the charismatic pirate Long John Silver to easily deceive him. As a result, Trelawney unwittingly hires Silver and a crew composed largely of mutinous pirates, setting the stage for the conflict that follows. His initial overconfidence also causes him to dismiss the legitimate concerns of Captain Smollett regarding the questionable character of the men he has recruited.

Despite these early failings, Trelawney is not a one-dimensional figure of ridicule. Following the mutiny, he demonstrates genuine humility by admitting his mistakes to Captain Smollett. As the situation on the island grows dire, Trelawney shows considerable growth and leadership. He coordinates the loyal crew members to overpower the pirates left aboard the Hispaniola and later joins Dr. Livesey and Jim Hawkins in a fortified stockade. In these moments of crisis, his true value emerges through his exceptional marksmanship. Trelawney is an extraordinarily skilled shot, with one notable feat involving a long-range musket shot fired from an unstable, overloaded boat that nearly kills a pirate gunner. He forms strong emotional attachments to his companions and openly grieves the death of his gamekeeper, Tom Redruth, demonstrating a capacity for loyalty and genuine feeling. After the treasure is secured and divided among the survivors, he returns to England with the remaining loyal crew, having successfully channeled his appetite for adventure into productive and responsible action.