Perhaps the OC is a pirate with a unique ability, like being able to communicate with animals. They meet Princess Vivi when their ship docks near Alabasta. Vivi is intrigued by the OC's ability and asks for help in solving a mystery regarding some strange animal behavior in the desert. Along the way, they face many challenges and their relationship evolves into a strong bond.
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.
Another storyline could be that the OC is a mage from a far - off land. They are drawn to Alabasta because of a premonition about Princess Vivi. When they meet, Vivi initially doubts the OC's motives. However, as time passes and the OC uses their magic to help the people of Alabasta during a drought, Vivi begins to trust the OC and they start a journey to find a way to permanently end the water shortages in the kingdom.
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 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.
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.
One possible storyline could be that Luffy and Monet meet during one of Luffy's adventures in a cold region. Monet, being a half - snow woman, initially sees Luffy as an intruder but then becomes intrigued by his fearless and kind - hearted nature. They might end up teaming up against a common enemy who is threatening the local people and their way of life.
One possible storyline is that Naruto is somehow transported to the One Piece world through a strange jutsu gone wrong. He meets Nami and they have to work together to escape from a group of pirates who are after Nami's map. Naruto uses his speed and strength to fend off the pirates while Nami uses her wits to outsmart them.
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.
Luffy would be the central figure. His relationship with Vivi is one of friendship and mutual respect. In the context of a harem, Luffy might not fully understand the concept at first. Vivi, on the other hand, could act as a mediator or an equal partner among the female characters in the harem. The harem members could all be united by their admiration for Luffy and his noble goals.
In a fanfiction, Uta and Luffy could team up to discover a long - lost island that is said to hold the key to a powerful and ancient musical instrument. This instrument has the ability to control the sea itself. Along the way, they face many challenges and their different outlooks on life - Uta's more artistic and emotional one, and Luffy's straightforward and goal - driven one - clash and then blend as they learn from each other.
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.