In an interview, the creator of the series, Eiichiro Oda, revealed his favorite character in One Piece. Although he is a huge fan of Luffy and most of the Straw Hat Pirates, his favorite character is none of them. Oda revealed in an interview with Dragon Ball creator Akira Toriyama that his favorite character is Gaimon.
I like Luffy. But the character I like the most is Gaimon.
It’s hard to think of anyone better than him. Just a man who’s been stuck in a treasure box for 20 years. I myself think it was an amazing setup.
Gaimon is a former pirate who currently lives as a hermit on Rare Animal Island.
In the manga, Gaimon’s debut is a relatively low-key event that takes place between the Orange Town Arc and the Syrup Village Arc, with Luffy and Nami being the only Straw Pirates he encounters (Zoro being too exhausted by fighting to go ashore). The anime places Gaimon’s debut between the Syrup Village Arc and the Baratie Arc, incorporating Zoro and Usopp into the encounter and expanding the overall story somewhat.
When he was young, Gaimon was a stereotypical greedy pirate, caring little more than plunder. This greed slowly eroded (although it was never completely erased) during his decades of solitude on the island of rare animals, leading him to place more importance on peace and camaraderie than to any material wealth.
Although he first tried to attack the Straw Hat Pirates, he eventually befriended them after telling them about his past, and they tried to help him get the chests at the treasure he wanted. However, with the chests empty, Luffy asks him to join his crew, but Gaimon declines the invitation to stay with the animals.