How is Zoro like Sasuke? Last time I checked, Zoro is Luffy’s underling and not his rival
Well I did not say their roles are exact same, obviously its a different story, but both are same in a sense that they represent serious/edgy secondary protagonists though I personally think Zoro is much better written character than Sasuke to me as he is an alpha male with high moral compass rather than an emo in literal sense.
My opinion on Zoro and Luffy is that idea of captain/underling is very fluid between them as their pair has never been presented as that of a captain and underling, instead two partners. The idea of partners has already been established in One Piece with Vice Captains like Killer and Rayleigh referred to as partners by their captains in manga and I think same logic applies to every true vice captain so that would include Beckman as well.
But with Zoro its even more than just him being a Vice Captain, he is truly an exception of a subordinate because Zoro joined the crew with his own ambition, he has will power, hunger and drive to be better just as much as Luffy, but their destinations are different. We can look at Mihawk's statement "Becoming pirate king is harder than me" to face value but reality is Zoro's own opinion of his ultimate destination is much different than that, to him becoming WSS is like his name reaching heaven, he will probably end up being far greater than Mihawk ever was, perhaps strongest swordsman ever. But the point is that the way both Luffy and Zoro see the world, their state of mind, they are two people on similar wave length with similar desire to be better but with different destinations. So I think even more so that Killer/Rayleigh, Zoro represents what a partner truly is to Luffy.