Gaimon

Gaimon (ガイモン) is a former pirate who lives as a hermit on the Island of Rare Animals. He first appeared in the manga between Buggy the Clown arc and the Captain Kuro arc, in the anime he appeared in between the Captain Kuro arc and the Baratie arc.

Appearance
Gaimon has a green afro hairstyle, a black beard, a chubby face and a big mouth. He has been stuck inside a treasure chest for 20 years.

Personality
He's hostile to those who'll try to hurt the animals or steal the treasure chest he's been guarding for 20 years (ironically, unbeknownst to him until the Straw Hats arrived, the chests were empty). But when he learned that someone who came into this island isn't interested in the animals or the treasure, he can become very friendly.

Past Story
Gaimon's life changed when he was searching for treasure on the island as part of an unnamed pirate crew. While searching, he fell from a high cliff and directly into an empty treasure chest at the bottom. His crew, considering him a lost cause, left Gaimon stranded on the island.

Over the next twenty years the treasure chest became a part of his body and could not be removed without killing him, however, he can still use his hands, head and feet. Because Gaimon shrank to fit the size of the small chest, he could no longer scale the cliff to the treasure. He managed to befriend the strange animals on the island, and developed a close bond with them. To protect his treasure and his 'nakama', he did not allow any visitors on his island. If any sailors come ashore there, he would hide and tell them to leave in a spooky voice, claiming to be the island guardian ("island god" in the anime). If this failed, he takes the more direct route of shooting them with a pistol (which he apparently keeps hidden inside his afro).

Present Story
Twenty years after he was first stranded, Monkey D. Luffy and his crew arrive on the island. They survive his deterrent tactics, and, after he realizes that they aren't there to steal from him, they become friends. However, when Luffy finally gets to the top of the cliff, Gaimon realized that the treasure chests were empty too and he had been protecting a nonexistent "treasure" for 20 years. Then, he considers the animals as his treasures, and vows to protect them.

Chibi Buggy eventually meets, and befriends, Gaimon in the "Special Cover Miniseries".

Anime and Manga Differences
The introduction to Gaimon is different in the anime and manga. In the manga, they haven't recruited Usopp and attain the Going Merry yet; and only Luffy and Nami met Gaimon, Zoro slept in the boat the entire time. In the anime, Usopp had already joined and they had the Going Merry; and all four of the Straw Hats met Gaimon.

Translation and Dub Issues
In the original Japanese version, there is a running gag in which other characters ask Gaimon if he's a "boxed son", in which he responds with "Yes, my father held me back when I was a boy" and then irritatedly snaps back to reality. This is a pun off of the Japanese term "boxed daughter", which refers to a girl who's been sheltered throughout her life. Since the joke probably wouldn't make sense to most American readers and viewers, Luffy instead calls Gaimon a "boxer" in the Viz Manga. The joke became a "Jack-in-the-Box" in the FUNimation dub. 4Kids turned the joke into "heart of gold" as a reference to the treasure normally held within a treasure chest.

Trivia

 * Eiichirō Oda has stated that Gaimon is his favorite character to draw.
 * Gaimon appears in Grand Battle/Rush! and Grand Adventure, not as a playable character, but as an item. If you hit him, he will spin around, and when he stops, he fires his gun rapidly in whatever direction he is facing. You can also pick him up and either throw him or press the Up Attack button to get him to shoot his pistol in the direction you're facing.
 * He also appears in One Piece: Unlimited Cruise as an enemy, and is playable in vs. mode.