They are also both key members of the Straw Hat crew. In fanfiction, their relationship can be explored in different ways. For example, Luffy's unwavering trust in Robin and Robin's respect for Luffy as the captain can be the basis for many stories. Their combined abilities, Luffy's physical strength and Robin's knowledge of the world, can lead to exciting adventures in fan - created stories. Also, the mystery surrounding Robin's past and Luffy's dream of becoming the Pirate King provide rich material for fanfiction writers to build on.
The main reason might be the contrast between the two characters. Harry is from a world of magic and mystery in Hogwarts, while Luffy is all about adventure on the high seas in the One Piece world. Writers like to explore how these two different personalities and power sets would interact in a shared story.
It's popular because it shows a different side of their relationship. People like to see Luffy being more than just his usual carefree self and using his smarts in relation to Nami. It adds depth to their dynamic.
Well, it could be that in certain fanfictions, the writers are trying to explore 'what - if' scenarios where Ace and Sabo have some sort of falling out with Luffy. It might be because of jealousy, perhaps they think Luffy is getting more attention or something. But really, this is a huge departure from their normal loving relationship in the main One Piece story.
It's popular because their relationship in the original story is really interesting. Luffy is the carefree and determined captain, while Robin is the intelligent and mysterious archaeologist. Fans like to explore more about their potential interactions.
They're popular because fans love the idea of Ace and Sabo being there for Luffy's upbringing. It shows a different side of their relationship. We usually see them as comrades, but in these fics, they're like family, which is really appealing.
In the One Piece world, Luffy is on this grand adventure and Hancock is a powerful figure. Their relationship has this allure of the unexpected. Fans are drawn to the idea of these two very different characters coming together. Fanfic writers can play with their personalities and create all sorts of interesting scenarios. For example, Hancock's initial haughty attitude towards most people but her special soft spot for Luffy makes for great storytelling. It's also about the idea of love in a world full of pirates and battles, which is quite appealing.
In episode 134 of One Piece, Luffy's interest in books attracted everyone's attention because of a special book called the "Sea Tower Stone Book" that appeared in this episode. This book could absorb the essence of humans and have the power to make humans superhuman. When Luffy was reading this book, he was attracted by the power in the book and tried to find more books to understand this power.
Luffy was interested in books because he was eager to learn more about the sea and marine life and hoped to become a great navigator. He hoped to improve his wisdom and ability to better explore the sea and the world by reading the stone book.