He incorporated ryuma a protagonist of another story in one piece through zoro. That doesn’t mean there are 2 protagonist or a deutergonist in the actual story just that zoro is based on one
It’s pretty obvious the second main character just depends on the arc. With it constantly changing up
Within the crew zoro’s definitely the strongest after luffy but he isn’t any more important than the rest of the crew
Zoro like oden, ace, or Yamato are characters that you wouldn’t expect to work under anyone. Something constantly highlighted about zoro to usually further hype luffy for having him as a subordinate
But yeah the story doesn’t have one
It’s very clearly luffy’s story with everyone else playing a side role. Depending on the arc the 2nd most important character besides luffy changes. The 2nd most important strawhat changes arc to arc too
Dressrosa for instance
- Luffy the main character obviously
- Law was pretty much the 2nd main character
- Usopp was the most important strawhat