With Robin, focus on her past and how it shapes her reactions to the OC. Her trauma from Ohara can make her initially guarded. But as the story progresses, the OC's presence can help her open up more. For Nami, emphasize her love for money and the crew. The OC can either support her in her money - making schemes or challenge her views, which develops her character further.
For the OC, give them a distinct personality trait. For example, if they are overly curious, it can lead to interesting interactions with Robin's love for knowledge and Nami's practicality. Show their growth over time, like learning from Robin's wisdom and Nami's street - smart ways.
Maybe the OC is from a long - lost island with unique knowledge. Robin is interested in the historical aspects of this knowledge and Nami sees the potential for great treasures. The OC then has to deal with the different personalities of Robin and Nami. Robin is calm and intellectual, while Nami can be feisty. Their interactions lead to a series of adventures and a deepening of their relationships.
Adventure is a very common theme. Since Nami and Robin are part of the Straw Hat crew in One Piece, many fanfictions involve the OC joining them on their adventures in the Grand Line. For example, they might go on a quest for a lost treasure or to discover a new island.
There's a fanfic called 'Fateful Encounters'. In this story, the OC meets Nami and Robin in a rather unexpected situation in a bustling port town. The author does a wonderful job of developing the relationships gradually. As the story progresses, they embark on a journey to find a hidden treasure, and throughout the adventure, the bond between the OC, Nami, and Robin deepens.
A different approach could be a fanfic where Nico Robin discovers she's pregnant first, and Nami later. Robin could be more calm and collected about it initially, while Nami is a bit more panicked. But as time goes on, they both grow into their new roles as future mothers. The story could also explore how their pregnancies change their relationships with the other crew members and their own personal goals.
To develop Rebekah, explore her past more. How has her time as a vampire affected her relationships? What are her hopes and dreams? For the OC, build their backstory gradually. For example, if they're a werewolf, how did they become one? How has it shaped their view of the world? Let their relationship develop organically. As they interact, they learn more about each other and change in response to each other.
One idea could be that the OC is a new pirate who joins the Straw Hats. Over time, they develop a special bond with Nico Robin due to their shared interest in ancient history. Meanwhile, during an encounter with Boa Hancock, the OC manages to gain her respect in an unexpected way, like saving her from a new threat in the Grand Line. This could lead to a complex love triangle where the OC has to navigate their feelings for both strong - willed women.
The theme of growth is quite prevalent. The oc might start off as a weak or inexperienced character and through their interactions with Robin, grow in strength and knowledge. Romance is also a common theme. Writers often explore the relationship between the oc and Robin, from the initial attraction to a full - blown love story. Additionally, the theme of redemption can be found in some fanfics. Maybe the oc has a troubled past and Robin helps them find a new path in life.
First, you need to develop your OC well. Give them a unique backstory, personality traits, and goals. For example, if your OC is a chef, their cooking could be a way to bond with Nico Robin (maybe she loves a certain dish) and Boa Hancock (by impressing her with a special recipe). Also, understand the personalities of Robin and Hancock. Robin is calm, intelligent, and has a dark past, while Hancock is proud, strong, and has her own insecurities. Incorporate these into how your OC interacts with them.
To write a great fanfic, start with a strong plot. Maybe there's a prophecy about their children being very important in the One Piece world. This would add an interesting layer to the pregnancy story. Develop the relationships between the characters well. Show how the other Straw Hats interact with Robin and Nami during this time. And don't forget about the details. Describe how they might decorate a nursery on the ship or what kind of baby clothes they'd get.
The combination of an original character (OC) with Nico Robin in a harem setup is interesting because it allows for a lot of creative freedom. Writers can create unique personalities for the OC that either complement or contrast with Robin's character, leading to interesting interactions.