At first, not very well. The Straw Hats have had bad experiences with Marines. They might see the Marine OC as a potential threat. However, if the OC shows that they are different from the typical corrupt Marines, like by helping the crew out of a tough situation right away, they might start to be more accepting.
Well, it could be really interesting. There would probably be a lot of initial distrust from the Straw Hats, since they are usually against the Marines. But if the OC has a good heart and a strong reason for leaving the Marines, like disagreeing with their corrupt ways, the Straw Hats might gradually accept them. The OC could bring in some unique skills and knowledge about the Marines' operations, which could be useful for the crew in various situations, like getting out of tight spots when facing other Marine forces.
It could be Chopper. Since Chopper is a doctor and Reiju has some medical knowledge, they might bond over sharing medical techniques and treating the crew's injuries. Also, Chopper's kind - hearted nature would make him a great friend for Reiju who might be used to a more cut - throat family environment.
If Naruto was raised by the Straw Hats, he'd probably be a very different person. He might be more carefree like Luffy. For example, he'd be used to the wacky adventures on the Grand Line. His ninja skills could blend with the pirates' combat styles, creating some really unique fighting techniques.
Well, as of the latest manga chapters, Yamato has indeed become a part of the Straw Hats. This decision brings new dynamics and adventures to the crew.
His personality would become more outgoing and adventurous. The Straw Hats' lifestyle is full of exploration, so Naruto would be more eager to take risks.
I really like the fanfiction where the Straw Hats time travel to the future. In this story, they see how their actions have affected the world in the long run. It shows a post - apocalyptic world that was caused by a series of events that they were unknowingly a part of. It makes them reflect on their current actions and how they can change things for the better. Another good one is the one where they go back to the time of Gol D. Roger. They get to meet the legendary pirate and learn some of his secrets, which is really exciting for fans of the series.
Well, one possibility is that Reiju could bring her unique skills like her combat abilities or her knowledge of certain technologies to the Straw Hats. Maybe she joins them to seek adventure away from her family's influence. She could also form interesting relationships with the crew members, for example, having a sister - like bond with Nami due to their shared intelligence in different areas.