John Trelawney is a kind-hearted but gullible country squire who, like Jim, is enthralled by the sea. He has two significant roles in Treasure Island. The first is that he kick-starts much of the novel's plot. He is the one who wants to set sail after Jim shows him and Dr. Livesey Flint’s map, and he both funds the trip and commissions their ship, the Hispaniola. Trelawney is also the reason why Long John Silver and the other pirates learn about the voyage—he could not stop himself from excitedly boasting about his upcoming adventure while preparing to set sail. Long John Silver and the other pirates are able to capitalize on Trelawney’s gullible nature and trick the squire into hiring them for the Hispaniola’s crew. Without Trelawney, the voyage and subsequent mutiny and treasure hunt may never have happened.
Squire Trelawney also has a more thematic role in the narrative. Trelawney is swept up in a boyish fantasy from the moment he learns of the treasure. He is clearly very interested and enamored with pirates and with sea-faring in general, exemplified when he excitedly teaches Jim and Livesey about pirate lore as they study the map. When discussing their upcoming trip, he thinks only of fine weather and strong ships and riches, never mentioning any potential threats or hardships. Trelawney has a romanticized, if not entirely unrealistic, vision of treasure hunting and approaches their adventure like an enthusiastic child instead of a rational adult. In that sense, he acts as a sort of mirror for Jim Hawkins, the only actual child in Stevenson’s novel. Squire Trelawney’s transformation from a responsible and wealthy gentleman into an adventurous boy again the moment that treasure is mentioned implies that everyone, regardless of their background, has a little pirate in them, which speaks to one of the novel’s key themes.