The OC can be developed by having them learn from Vivi's strong sense of loyalty and duty to her kingdom. For example, if the OC was a bit of a carefree spirit before, seeing Vivi's dedication could make them more responsible. They could also develop their fighting skills by training with Vivi, who is no stranger to combat situations in the One Piece world.
The OC's character can be developed through conflict with Vivi as well. For instance, if the OC has a different view on how to handle a certain situation in Alabasta, like dealing with a group of bandits. Vivi might want to use diplomacy, while the OC thinks a more forceful approach is needed. Through this clash of ideas, both characters can learn from each other. The OC can become more understanding of the political and social implications of their actions, while Vivi can learn to be more flexible in her decision - making.
One way to develop the OC in relation to Vivi is through shared experiences. If they go on a journey together, like exploring ancient ruins in Alabasta, the OC can grow in knowledge and wisdom just as Vivi does. Also, the OC could be influenced by Vivi's diplomatic skills. Maybe the OC was not good at handling people, but by observing Vivi's interactions with different factions in the kingdom, they become more adept at social situations.
One possible storyline could be that the OC is a wandering adventurer who saves Princess Vivi from a new threat in Alabasta. They then team up to discover the source of this threat which turns out to be an old enemy of the kingdom who has returned with a new power. As they journey together, they develop a deep friendship and respect for each other.
First, give the OC a strong motivation. For example, if they want to protect Alabasta like Vivi does, it gives them a clear goal. Second, a unique appearance helps. Maybe they have a distinctive hat or tattoo. Third, give them a special ability, like being able to control sand in Alabasta - style. These elements combined can make the character compelling.
The OC could be a healer with a strange and rare form of medicine knowledge. In the fanfic, the crew gets injured during a big battle and the OC uses their skills to save everyone. Their medicine is based on a plant only found in Alabasta. Vivi then realizes that the OC's presence can be a great asset to the crew. The OC also has a dream to spread their medical knowledge across the world, so they join the crew on their journey. As they sail, they encounter different cultures and the OC learns new healing techniques, sharing their own in return.
One way could be by focusing on their different personalities. Luffy's carefree and determined nature, Vivi's strong sense of loyalty and leadership. In the harem aspect, other female characters might be added. For example, Nami could be part of it due to her long - time relationship with Luffy. They could all go on adventures together, with Vivi guiding them in some aspects as she has knowledge of the Arabasta kingdom. The story could also explore how they all deal with the power of the One Piece world, with Luffy's goal of finding the One Piece at the center.
The popularity comes from the unique relationship between Luffy and Vivi in the original 'One Piece' series. Their friendship is very special, and fanfics allow fans to explore it further.
There's also 'Luffy and Vivi: Reunion'. This fanfic focuses on what happens when Vivi reunites with the Straw Hat crew after a long time. It beautifully captures the emotions and the unbreakable friendship between Luffy and Vivi, with some really exciting action scenes in between.
An OC's Haki can be developed through intense training. Just like how Luffy and the others train in the One Piece world. They could face difficult opponents and learn from each battle, gradually strengthening their Haki. For example, if they constantly fight against Logia - type devil fruit users, their Armament Haki would improve to counter their intangible forms.
His character can be developed by having him face moral dilemmas. In the One Piece universe, there are many gray areas. If the harem has different views on a certain situation, like whether to steal a valuable item from a corrupt noble. The OC male has to think about his own values and grow as a person. This way, his character becomes more complex and interesting in the fanfiction.
To develop the OC's character, consider giving them a backstory that ties into their relationship with Elsa. If the OC has a past trauma related to isolation, just like Elsa, it can create an instant connection between them. As the story progresses, the OC can learn from Elsa's journey of self - acceptance and apply it to their own life. This growth in the OC's personality, from a broken - hearted or troubled individual to someone who is more whole and at peace, can be a great way to develop their character.
You can start by giving the OC a distinct backstory. For example, if they're a vampire, explain how they were turned and what their human life was like. This will shape their personality and values.