@Razh:
So much that he made Urouge say it.
http://mangasee.com/manga/?series=OnePiece&chapter=499&index=1&page=16
http://onepiece.wikia.com/wiki/Mythbusters/Misunderstandings_and_Mistranslations under "Zoro the first mate". http://apforums.net/showpost.php?p=881217&postcount=41 Page 16. While not exactly being called First Mate/Vice-Captain but "Second", at that arc I do agree that Zoro was already the First Mate, which I'll try to explain.
@Lao:
I wondered recently, are Vice Captain and First Mate different rankings in a crew? I'm quite confused, because if, and that's a huge if (don't want to bother with the useless argument) Zoro is the First Mate in the Straw Hat crew, does this also indicate that he is the Vice Captain? The only one in the crew who gave me an impression of an actual Vice Captain was Usopp. So what is the difference between a Vice Captain and a First Mate?
First off, do you guys remember the instances where Usopp became shocked at the fact that he can aim well? Yeah that's important. I only need to post about Zoro and Usopp's but I finished the list recently so I might as well.
I made a list awhile back on how each SH crew members and their arc villains both have similarities with both Luffy AND their arc villains. This is important because it's related to the crew number chapter titles. If you're confused, look at the manga chapter titles there are:
"The First"/Zoro
"The Second"/Nami after Arlong Arc
"The Fourth"/Sanji
"The Third and The Seventh"/Usopp and Franky after leaving Water 7
"The Eight"/Brook
BUT there's no "The Fifth"/Chopper and "The Sixth"/Robin, not yet anyways.
Each of the Strawhat crewmate received their crew number chapter titles after discarding/putting aside a their "baggage".
For Zoro, he has a great "ambition" to be the World's Greatest Swordsman that he promised to his fallen friend. He was known as the 'Pirate Hunter' yet he joined with Luffy and became a pirate himself. Then later, he was willing to throw away that said "ambition" and sacrifice himself all for the sake of his Captain.
For Sanji, his "resolve" on staying on Baratie for what he thought was right to pay Zeff back for saving him. In the end he put that aside, in shock of everyone in Baratie and asked Luffy himself that he'll go with him on the journey to fulfilling their dreams.
For Nami her "belief" that all pirates are all alike. Arlong used her for his own benefit but Luffy not being your typical pirate, or even as a capable person said very clearly that he knows he needs others to keep on living. While it may be similar, it's also very fundamentally different. In the end she decided to follow her dreams and became a full member of the crew.
For Usopp, the reason why he got his chapter title at the end of Water 7 and not his arc, I believe is because he was too "proud". They left his island with the Going Merry which they got with his help. He settled for Sniper (first), despite his own surprise at his skills as a marksman and shortly after became Vice-Captain (second) of the Going Merry. His doubts, attachment, growing inferiority complex with the crew and his "pride" however associated in his mind that the Going Merry was his whole place in the crew - which eventually led to the fight with Luffy. The Going Merry's life, with him having to take care of her then finally her death with some loneliness and regret was very similar to his Mother's. But just the same, her death helped him to grow and focused on what was his right position in the crew, the sniper - as well as a new bond and dream, in which he vowed to be the REAL Sniper King to help Luffy. At the end of Water 7 we saw Usopp thinking of ways to talk to the crew saying he's willing to be the "Vice-Captain" again aboard the Sunny too, but that was short-lived (which is also why Zoro is probably the Vice-Captain aboard the Sunny). In the end though he threw down his "pride", apologized and begged them to take him back.
For Franky, it was the "blame" he put towards himself after creating weapons that was used to hurt innocent people and in framing Tom, sealing his fate (and possibly even before that - being abandoned by his parents). The last weapon he created was re-creating himself. Becoming the head of Water 7's underworld to give his fellow outcasts a place to belong like he had. Making a living dismantling ships and as bounty hunters - was a way to redeem himself for what he's created and to protect the city that his Master loved and gave life to. As a way to make amends to the Strawhats, he build them a ship, not just for atonement but also because he couldn't stop his dreams. In the end, he finally saw that continuing to create was what Tom wanted for him. With Tom's, then Iceburg's forgiveness and after his own men and family wishing for his happiness by seeing him off the island - he was finally able to forgive himself. He became a pirate's shipwright aboard his own creation and sail it to fulfill his dreams - but also maybe to surpass Tom.
For Brook, the "loneliness" he felt being alone for 50 years after watching his crew dying is too great for anyone, but Luffy invited him to join the crew despite his appearance and the crew putting their life on the line to help him regain his shadow. With his spirit to keep on living, the last song of his crew and knowing that Laboon's still waiting, he joined and swore his life to Luffy.
By the way, the arc villains I mentioned are:
Former Captain Morgan's "ambition" for a higher rank
Don Krieg's "resolve" to enter/conquer the Grand Line
Arlong's "belief" that humans are inferior creatures
Captain Kuro's "pride" regarding his plans
Spandam's need to always "blame" everything and others
Moriah's "loneliness" after his crew was wiped out by Kaido
The missing ones would be Wapol for Chopper and Crocodile/Lucci for Robin.
Anyways in my opinion Usopp WAS the Vice-Captain of the Going Merry. He settled for being a Sniper but he clearly stated he'll be the one in charge if Luffy ever "chickens out" and Luffy agreed to it. Usopp took care of the ship and is also a jack of all trades. http://onepiece.wikia.com/wiki/First_Mate His fighting style is with his slingshot. However, after he left the crew (and Merry's death soon after), it's no coincidence that Sniper King was born and his sniping skills was emphasized - with the chapter title "The Legendary Hero".
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Zoro on the other hand very clearly stated in the beginning, he would have Luffy's death if he ever got in the way of his ambition.
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So yes, in my opinion the greatest and most emphasized feat Zoro has ever done to make it clear that he is finally the First Mate aboard the Sunny is of course, back in Thriller Bark. The title of the chapter is "Strawhat Crew - Pirate Hunter Zoro".
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