John Trelawney is a typical example of the cheerful country squire. His name is an old one in parts of England, yet common enough apparently that Stevenson felt able to use it without seeming to refer to anyone in particular. He belongs to the landed gentry, is lower in social rank than those with hereditary titles, yet definitely of a higher class than Jim Hawkins' family, any of the villagers, and even Dr. Livesey (who is Trelawney's social equal because of education and manners, but does not own inherited wealth). Class and breeding are important to a man like the squire — far too important for him to flaunt his. Stevenson does not specify his age, only that he cheerfully calls himself an "old" bachelor, which probably means that he is well out of his twenties but not dangerously close to fifty or even the middle forties. He will marry when the right time comes, and will probably choose a woman much younger than himself, if only to hand on his name and estate to a son. He is a sportsman — his most trusted servant is his head gamekeeper, and he is a good shot and has a cool head when necessary. He has what his own time would have called a sanguine disposition: He is optimistic, friendly, believes the best of people, is not in the habit of worrying. He can afford not to worry; he can pay people to do that for him.
Trelawney's function in the novel is simple: He exists, first, to finance the treasure hunt, and second, to be the gullible, garrulous ship owner who will allow Silver to direct the whole venture by hiring a crew of pirates and a useless first officer. Does he — or does anyone else — learn from his mistakes, become a different person, develop at all? There is no evidence for this in the novel.
Yet when Stevenson's biographer Ian Bell says that "personality" in this book is "dispensable," he is not quite correct. Character is dispensable; such as it is, it exists only to further the plot. But character and personality are not exactly the same thing. Trelawney, for example, has plenty of personality, which is all he needs to make the reader believe in him, for readers do believe in him and never question his motivation. In great part, the reader's belief in Trelawney is due to Stevenson's gift for language. Not a word comes out of Trelawney's mouth or emerges (in his letter to Livesey and Redruth) from his pen that could have been said by any other character in the book. From his very first remark ("Mr. Dance, you are a very noble fellow. And as for riding down that black, atrocious miscreant, I regard it as an act of virtue, sir, like stamping on a cockroach" — this without ever before having heard of Pew) to his last ("John Silver, . . you're a prodigious villain and imposter — a monstrous imposter, sir"), he is blustering, overstated, self-important, and rather silly. In fact, Silver's having directly or indirectly caused the deaths of nineteen men angers Trelawney, but not much more than his having presented himself as an honest seaman and fooled Trelawney. Thus, although you cannot know the squire as a character in his own right, you do know him as a personality, true to his type.