Gaimon is a character that gets his body imprisoned in an empty treasure chest after he loses his grip while scaling a small cliff on the mysterious Island of Rare Animals (Gold Island in the 4Kids dub) trying to find some treasure with his pirate crew. The crew leaves Gaimon stranded on the island. The treasure chest becomes his body and cannot be removed, however, he can still use his hands, head and feet. It takes 20 years for another person to find him.

After Gaimon shrinks when he gets stuck in the chest, he can no longer scale the cliff. He has to protect his treasures somehow, so he would use his gun and his spooky voice to drive away intruders. Monkey D. Luffy and the whole crew passes the trial and they begin to 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. He was offered to join the crew of the Strawhats but he declined. 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."