Ben Gunn is a man who has been marooned on Treasure Island. He had been one of the pirates along with with Long John Silver. The pirates left him on the island because he really wanted to find Flint's treasure and the other pirates were tired of looking for it.
By the time Jim finds him, Ben Gunn has gone a little bit crazy. But he is the one who makes it possible for Jim and the others to survive. He helps them by giving them food and letting them use his boat.
We are never really told why Silver hates Trelawney. It is probably because Trelawney was born rich and important and Silver was not.
Jim jumps in the boat with the pirates and goes to the island just because he feels like it -- no real reason.
In "Treasure Island" by Robert Louis Stevenson there are some unique characters. Long John Silver who is acting as a cook on the ship, Hispaniola, knows that Mr. Trelawney was once a pirate, and that he knows about Flint's treasure.
Ben Gunn was a pirate that was marooned on a desert Island. It was a common practice among pirates. When the pirates arrive to steal the treasure they hear someone singing. It is Ben Gunn. No pirates are afraid of him. He was smart enough to hide the treasure. In the end of the story he has part of the treasure but he spends his share too quickly, within three weeks. He eventually buys and manages a lodge house. He is a skinny white man, fairly attractive, and sunburned. His lips are black and his skin is very dark from sun exposure. He looks like a beggar in ragged clothing. He is wearing clothing made from a ship's canvas.
“and this extraordinary patchwork was all held together by a system of the most various and incongruous fastenings, brass buttons, bits of stick, and loops of tarry gaskin. About his waist he wore an old brass-buckled leather belt, which was the one thing solid in his whole accoutrement."
The doctor is visiting the pirates who have been held. Long John Silver takes Jim and lets the doctor know that he will release him. He uses Jim as a bargaining chip so he can see the squire and Jim can report how Silver had protected him. He believes this will save him from hanging.
"was roundly accused of playing double—of trying to make a separate peace for himself—"(Chapter 30)