He keeps the straw hat with him.
Sure, it gets in the way sometimes. And the string can get annoying around his neck like that, but if he removes it--if he loses it because it wasn't on him--its owner would kill him. He's only watching it for him, after all. He's careful not to alter or damage it in any way. He's even extra careful when he checks that Ace's viver card is in still place every few days.
Of course, some things have happened here and there along the voyage. He has a lot of enemies, after all, by now they all know who he got it from, and they know how much the hat must mean to him.
He takes it with him everywhere he goes, his constant reminder of why he must move his nakama forward. If he didn't keep his word, he knew who he'd have to answer to.
The Mugiwara pirates are infamous around the world, and simple straw hats are rarely vended at islands they've never been to. But they sell like hotcakes at all the islands they've saved. Even Skypiea. Oh, yes. They've been back.
He insisted on searching everywhere, after all. And according to his logic, who wouldn't want to hide in heaven? Especially if…
Sunny is in great shape for all she's been through, and Frankie never lets any of them forget it. Nami's map is coming along nicely despite all the setbacks. He makes sure of that. He's led Usopp to discover the brave man he's growing into. He was leading when Chopper discovered the cure for both Kaleidoscope Disorder and Pisces Nocturne disease. He leads while Robin continues to seek out the hidden ponoglyph that will answer all her questions. He leads Sanji as the cook becomes disheartened by his failing quest to find his dream sea. Sanji is perhaps the only other one being held back by what happened. Then, of course, there's Zoro, but Zoro waits because he has a question of his own, and he can't move forward himself until he has the answer. He won't.
But he still leads them.
Because still no one holds the title of Kaizoku-ou. Because still no one rules the seas. Because it would make him proud. Because he made a promise.
He's been searching for six years, even though the crew has told him it's hopeless. Even though he's read the papers himself and there has been no indication that he's even still alive. Even though the marines have since rescinded the bounty on his head.
Because he still has hope. Because they never found a body. If they had any concrete proof, they surely would have gloated about it and invented some heroic story about how they, themselves, had taken down the great pirate menace.
Sure, it could be as Nami says and his body is in the sea now, as all great pirates' were. He would have liked that. He would never have settled for anything less. But every now and then there are sightings; people who claim to have seen him walking along beaches of the Grand Line as if looking for something that belonged to him, or standing and looking out to sea as if waiting for something to happen--for someone to come.
The crew has had yet to verify one lead, and most of them think they're lies.
…He hopes they've been lies. Because, as Usopp has pointed out, the only real possibilities that would make them truths are more painful than not knowing at all.
But if they are lies, that means that somewhere he's doing an amazing job of getting through the days without creating havoc in whatever island he's on, and when they find him he's going to get scolded for that. Discretion had certainly never been in his vocabulary before he'd last seen him!
But that was a long time ago now.
And deep inside, he knows, he knows what the unnatural silence probably means. But not finding a body… that means something too.
And that's why he still questions. That's why he can't let go. That's why he won't change their flag.
Like the keeper of the hat before him, he's never been one to accept a loss.
He sits on the figurehead sometimes, trying to remember the world as it had been before, trying to see in the horizon the same adventures that had used to be there. The same ones that he had always seen. And some days he feels that he's beginning to forget what life was like before they'd been separated in the flooding storm when the cannonballs had started to hit Ravenden Island.
Six years have passed, and each time they reach a new island on the Grand Line, going deeper and deeper in, he still leans over the rail and for an instant, he sees him on the crowded dock, watching them moor with that huge smile and those twinkling eyes, as if he'd been standing there waiting for an age.
But then he's gone, and once again his heart clenches in disappointment, hoping that at the next island it would happen. He would be there laughing as they jumped off the ship, full of excitement and satisfied longing, saying "Wahoo! There you are! Geez, making your captain wait so long. Hey! You kept it safe for me! Thank you, Zoro!"
He won't comment on how well he'd led their nakama in his stead, because he expects nothing less.
But until that time comes, until he has a definite answer, he will wait to confront Mihawk again. He will wait until the day his captain can watch him. And he will continue to lead them. His captain never loses hope, and Zoro is determined to follow that example.
But some nights when he's in his room he swears he hears laughter ringing out above him, and excited footsteps running across the deck over his head, sandals slapping against bare feet on the upstep. And in those moments he believes he's being told in Luffy's own way the answer to his question.
But he keeps the straw hat with him, just in case.