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.
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.
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.
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.
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.
A luffy x nami x robin fanfic often explores the complex relationships among these three characters from 'One Piece'. It could show their different interactions, like Luffy's carefree nature affecting Nami and Robin in unique ways. Maybe it delves into how they support each other during their adventures on the Grand Line. For example, Luffy's determination might inspire Nami and Robin to be braver in the face of danger.
Often, these fanfics highlight their strong female friendship. They show how Nami and Robin, being two intelligent and capable women in the Straw Hat crew, support and complement each other.