Ben Gunn is a marooned sailor Jim Hawkins finds on the island. He had been a part of Captain Flint's crew, like Long John Silver and Billy Bones. But unlike these men, absolutely no one respects him. When Tom Morgan and George Merry hear a ghostly voice in the forest, they are terrified – that is, until they identify it as Ben Gunn's. George Merry exclaims, "Nobody minds Ben Gunn [...] dead or alive, nobody minds him" (32.30). In other words, even if the voice belongs to a ghost, it's Ben Gunn's ghost, and nobody should be concerned about that.
This gives you a sense of exactly how little respect everyone in the world has for Ben Gunn. In fact, as far as we can tell, he was basically abandoned on the island because he was annoying. He was sailing with a ship that happened to pass the island three years before when he promised everyone Captain Flint's treasure. The ship obediently stopped at the island and the crew spent twelve days looking before getting fed up and leaving Ben there with pickaxes to keep searching on his own.
Still, even if Ben Gunn isn't exactly an impressive figure, he is a survivor. Against all odds, he manages to live for three years on the island by himself, killing and preserving goat meat and looking tirelessly for Captain Flint's treasure. And he even manages to find it, two months before the Hispaniola even gets there. While Ben Gunn is too afraid to confront the pirates on the island by himself, he is wily enough to make powerful friends – Doctor Livesey, in particular – who give him free passage to England and a thousand pounds in exchange for news about the treasure and help in tracking the pirates. It goes to show that you don't need to be a courageous, impressive person like Long John Silver to change the fate of other people – you just have to be lucky and cunning.
A last note about Ben Gunn: when the poor man first meets Jim on the island, he tells Jim that spending three years on the island has made him a God-fearing man again. He was raised to believe in God as a child, and years alone have brought him back to faith again. So Ben promises that, if he returns to England, he won't fall back into his old pirate ways. Well, Ben may not become a pirate again – he's not popular or tough enough for that, we think – but at the end of the novel, he does completely go back to his old habits of drinking and spending too much money. He spends the thousand pounds Doctor Livesey gives him in nineteen days. On the twentieth day, Ben Gunn is begging again.
Not only does this prove how weak-willed Ben Gunn is; it also proves Long John Silver's earlier claim that the problem with all pirates is that they play as hard as they work:
Here it is about gentlemen of fortune. They lives rough, and they risk swinging, but they eat and drink like fighting-cocks, and when a cruise is done, why, it's hundreds of pounds instead of hundreds of farthings in their pockets. Now, the most goes for rum and a good fling, and to sea again in their shirts. (11.7)
Long John Silver explains that the life of a pirate is hard and painful, but once the journey is done, the payoff is great. The problem is that, with money in hand, most pirates go for rum and fun times on shore. So by the end of it, they have nothing to show for their crimes, and it's back to the rough life of the sea.
Ben Gunn proves that he's a pirate through and through, even if he isn't tough enough for the crime part, because he makes a huge amount of money, spends it all, and winds up a poor man with nothing to show for his adventures. The main problem with the criminals in this book is that they are so self-indulgent. If they'd been a little more disciplined (and sober – these guys drink all the time), perhaps Doctor Livesey and Squire Trelawney wouldn't have won out in the end.