When Zoro first joins Luffy's crew, he doesn't expect to be there long.
Yes, he intends to serve his captain with loyalty because that was what he promised. But he imposed his condition—do not interfere with my goal of becoming the world's greatest swordsman—for a reason. Sooner or later this boy pirate (seventeen is a boy to Zoro, who is a whole two years older) will get tired of Zoro challenging ridiculously strong people to fights or not showing up in the right place, and then he will do something that breaks their agreement.
Luffy will probably renounce his captaincy instead of killing himself as promised, and Zoro will let him because it's not his business to enforce someone else's honor. Wouldn't be the first time it's happened—Zoro is alone in no small part because he doesn't play well with others.
He doesn't mind calling the straw hat boy captain as long as it doesn't conflict with his own agenda. He won't stay a second longer once it does.
(He decided on his own that he was first mate, on account of joining the crew first. He doesn't feel the need to tell anyone else because it's too obvious.)
Straw hat has a good eye for talent, Zoro has to give him that. Nami is clearly a phenomenal navigator, a clever survivor, and woman with the heart of a pirate whether she cares to admit it or not. Perhaps Luffy might have the makings of a captain after all.
Once he gets to know Nami on a more personal basis, Zoro realizes that she's also money-crazy witch; it's a mystery what Luffy was thinking when he recruited the most demonic woman in East Blue.
(The irony escapes him.)
Zoro's testing and acceptance of Usopp does not occur until considerably after the sniper joins the crew.
It's not that Zoro disagrees with Luffy's invitation. No, the swordsman sees potential in the long-nosed boy and does not begrudge the time it takes to nurture it and to maybe protect him a bit more than the rest of the crew needs. He's not really interested in maximizing fighting power at that point because he still doesn't care that much, and still more than half expects to be gone in a month.
(Zoro has a soft spot for children, which would surprise anyone except perhaps a pig-tailed girl who lives on the island formerly ruled by Marine Captain Morgan.)
No, the first time Zoro fully and completely accepts Usopp is when he is screaming an apology as the ship sails away, and grabs on to Luffy's outstretched hand.
While Zoro is unconscious from his mind-numbing failure with Mihawk (and now carrying a second promise to never lose, one to Kuina and one to Luffy) the crew obtains a cook. He is a tad offended that this decision was made without his input—some testing is in order.
One above-average fight later, and Zoro decides that Luffy has brought him a perfectly acceptable sparring partner and second mate.
(It's lucky that Sanji can't read minds, or he might have left them then and there.)
At Whiskey Peak, Luffy attacks him and he fights back. Luffy may be captain but Zoro doesn't accept defeat for anyone.
The battle ends with no conclusion and they acquire a princess.
(Zoro is unsure who is stronger and even more unsure as to why he isn't itching to find out.)
Luffy leaves with a sick navigator on his back and comes back to the ship with a reindeer who Nami calls a doctor. A little bit of testing is in order—if Luffy says he is crew, then he is, but given that the captain didn't even know he was picking up a doctor, they'd better find out if their new crewmember is fit for the job or should be just considered another fighter.
Zoro allows a weight to fall on his toe and goes to the sick room for assistance. The reindeer fusses and uses out a funny green paste. Then he treats several other injuries Zoro hadn't come for.
His body feels a little more invigorated when he practices that night, and he checks off the box for doctor in his head.
Zoro doesn't expect Nico Robin to stay.
He can see right away she has an emptiness in her heart and she only came to Luffy because he'd obligated himself to take her. Once her goals no longer lie with him, she'll be gone. He doesn't necessarily think she will betray them, just that their paths will soon diverge. She can win over the others easily and it doesn't bother him, because he will stay on his guard.
(Then he forgets, like everyone else, on Skypiea.)
But Luffy says he wants Robin back, and looking into his eyes Zoro also believes that Robin hasn't betrayed them. As long as the captain wants her she is one of them.
(Enies Lobby burns in their wake.)
Franky was inevitable—Zoro saw it before they even left Enies Lobby. He's a cyborg. He has a dream and his skills are at the top of his field and he's as crazy as the rest of them. He slipped into the crew before he was crew, and Zoro knows Luffy well enough to know that it's easier to get in than out.
(He's starting to understand Luffy's criteria for crewmembers.)
When Luffy comes off a ghost ship with a new nakama, Zoro is less than thrilled. But he also recognizes that it's a lost battle from the start. If somewhere in the world there is a walking talking skeleton, with an afro and music and the yo-ho-ho, then Luffy will have him in his crew. He would consider it a disgrace for such a person to go to anyone but the Pirate King.
"Good! Become the world's greatest swordsman. Since you want to become the King of Pirate's crew member, if you can't do something that small, I'll be disappointed!"
Fortunately, this skeleton also has the true spirit of a swordsman. Zoro takes over the job of retrieving his shadow without being asked. It's his role as first mate.
Then comes the enemy he cannot defeat. Zoro knows the truth in his bones from the instant he sees Kuma move.
"Let my life exchange for his! Eventually I will become the world's number one swordsman!"
Letting Sanji do it is no improvement, so he makes sure the cook stays down this time.
"Luffy must be the man who becomes Pirate King."
(Once, he'd have killed Luffy for the sake of his dream. Now he will sacrifice his dream for Luffy, for any of them, because they are all his nakama.)
He is Roronoa Zoro, first mate of the Straw Hat Pirates, nakama to Luffy, Nami, Usopp, Sanji, Chopper, Robin, Franky, and Brooke. He carries all of their dreams on his back. He is the man who will become the world's greatest swordsman and right hand to the Pirate King.