Making Zoro a Conqueror and a King, but in the meantime someone who as strong and as indipendent as he is still willingly decides to follow someone else, the author showed how great is Zoro’s devotion to Luffy.
See? Exactly like Whitebeard wondered how Oden could join him, as he is the kind of guy who leads, not the one who follows others. Also Whitebeard made a reference to the many Conqueror Haki users in Rocks Crew, the same thing to which Kaido will make a reference as well, about how Conqueror Haki users aren’t people prone to follow others.
No wonder that then when people see Zoro they immediately think that the swordsman should be the leader of his own crew or the leader of the Straw Hat.
But that doesn’t matter, Zoro prefers to follow Luffy. Why? Very differently from the others, Zoro’s strength is extremely close and comparable to his captain’s one. Zoro doesn’t follow Luffy out of need, like the others, but because of the spontaneous trust that he has in him.
Even if, as already showed, Zoro is the kind of guy who dominates, not the one who follow others. It’s just that his devotion to Luffy is that intense. More than a Right Hand Man, more than a First Mate; more like a brother.
That’s also why Oda had the two of them share so many parallels: strength, power ups, backgrounds, goals, benchmarks. Luffy and Zoro make the best duo ever. Captain and Right Hand Man/Number Two/First Mate, for life.
Like it or not, the amount of different times and different ways where Zoro is put close to Luffy is gargantuan. The sheer number of those comparisons is astonishing, it screams loud, like the absolute absence of the same closeness between Sanji and Luffy, screams loud as well
… but in the opposite way, because there’s zero of them.
No wonder that we have this.
You all know that the strongest Straw Hat bar Luffy is Zoro. And if by chance you didn’t know… Oda just stated this in SBS 102. No equals… just an undesputable number two which is Zoro, with no one close enough to make this ambiguous.