One way to develop the characters in a One Piece game story is by putting them in moral dilemmas. For example, Nami might face a situation where she has to choose between saving a village from a corrupt pirate or getting a valuable map that could lead to the One Piece. This would test her character and also allow her to grow in different ways.
In fanfiction, they can be developed through their interactions with other characters. If they meet a character who challenges their beliefs, say about loyalty to Law. This could cause them to question and grow. Also, putting them in situations where they have to make tough decisions like sacrificing their own desires for the good of the crew would show their growth. For instance, if they have to give up a valuable item they found for the safety of the whole Heart Pirates.
Female characters can be developed by giving them unique skills. For example, a female character could be an expert in navigation, which is crucial in the One Piece world. This skill would make her an invaluable part of Law's crew and also give her a sense of purpose and identity within the story.
In fanfiction, Sabo's character might be developed to show more of his softer side towards Koala. For example, in the original he is very focused on the revolutionary cause, but in fanfiction he could be shown taking time for Koala and having more personal conversations with her.
One way is through their relationships with other characters. If there's a store owner and an assistant, their relationship could start off rocky but develop into a strong partnership over time. This development can be shown through how they interact during busy hours, when they have disagreements about inventory, and how they support each other in difficult situations.
Sanji's chivalrous nature can be further explored. For example, when he and Usopp are in a dangerous situation, Sanji could put himself in harm's way to protect Usopp, which shows his loyalty and sense of honor. Usopp, on the other hand, can develop his courage. Maybe he stands up to a powerful enemy to protect Sanji, overcoming his usual cowardice and growing as a character.
One way is through character backstories. By delving into the pasts of the OCs, you can create connections that lead to the polygamous relationships. For instance, if two OCs have a shared trauma from their childhoods, it can be a reason for them to form a bond in this unique relationship setup.
Viola is a strong - willed and perceptive character. The male reader's personality could be developed to complement hers. For example, if he is more of a carefree spirit, his positive attitude could influence Viola to be a bit more relaxed at times. And in turn, Viola's seriousness can make the male reader more responsible. They can also have shared values like a love for justice or protecting the innocent, which would strengthen their bond as they face challenges together in the fanfiction.
The main characters are likely to be the members of the Straw Hat Crew, like Luffy, Zoro, Nami, etc. They are the central figures in the One Piece world and would be in the game's original story as well.
Explore their relationships more. Maybe they have a long - lost family member who is a key figure in the One Piece world. By delving into this family connection, we can see different sides of the supernova character, such as their loyalty, love, and determination.
Their relationship can be developed through shared adventures. For example, when facing a really tough enemy, Luffy's determination to protect Robin and Robin's trust in Luffy's strength can bring them closer. Each time Luffy saves Robin from danger, it deepens her respect and loyalty to him.