It would never have occurred to Luffy to ask who it is.
He doesn't care about the past, after all, to the point that he'd rather take a walk than hear about it. Luffy believes—and he shares this belief with those who ask—that his crewmates are here with him now, and that they were not before is beyond irrelevant.
Enies Lobby had changed that, to a certain extent. It was Robin's past that trapped her, pulled her away from life on the high seas with the people who called her family. Really, though, that had only been the latest event to shake Luffy's faith in the power of the present. Arlong, too, had been a relic of personal history that unfairly haunted his crew.
So, really, it's not like the question wouldn't have occurred to him before today, but more like the answer never would have seemed worth the asking.
There had been darkness in Robin's eyes before she left, as there had been before that in Nami's. If Luffy had thought to ask about it, could they have avoided all that pain?
A troubling thought to one not overly prone to reflection.
And of course it is an absurd notion that Zoro would turn from him for any reason, no matter the depth of his darkness. Although, Luffy has never truly had all of him; Zoro has his own ambitions that he follows with immense determination.
Perhaps it is not quite right to think of it as a matter of possession, but that is rather close to what it feels like: that Sanji and Nami and Chopper and the all others had given him all of their lives to him freely, regardless of the past and regardless of the future. Zoro still withholds; not about Mihawk, but the someone-else.
It wouldn't be a bother at all, Luffy has decided, if only he knew what was being withheld. If only he knew it did not threaten his apparently tenuous claim on whatever part of Zoro's heart he can call his own. Hence tonight.
The two are at a pub, and Zoro is downing rum after rum with his usual abandon. Luffy had a drink, some hours ago, but prefers alcohol when there is something to celebrate or promise. He has long since depleted the pub's stores of meat, and the others returned to Sunny some time ago.
The bartender, a nervous, codlike man, sidles up to the future Pirate King. "Could you tell your friend I have to cut him off?" he mutters, not wanting to tell the former pirate hunter he won't give him more booze.
Luffy offers the man a grin and calls down the bar to his swordsman. "We're leaving, Zoro!"
Zoro grunts something affirmative in reply, finishes his drink, and stands with only a little unsteadiness. Luffy wraps a friendly rubber arm around the bigger man's shoulders and guides him out firmly.
Outside, in the salty breeze, Zoro relaxes against the solid warmth of his captain. "Thawas fun," he says, voice slurred. He has allowed himself to get drunker than normal.
"Yup," Luffy agrees.
They are silent for a while as they make their way toward the docks. Luffy suddenly, and with the instinct he's come to trust, knows that Zoro is not thinking about Luffy, or the party, or even the hell he'd get from Nami for overspending, but rather that mystery darkness that called the swordsman's heart its own.
The question comes out in a rush. "Who is it?"
Zoro is suddenly alert. "Who is what?"
"The person you dream about." It is a guess, of course, that Zoro dreams of his darkness-person, and not necessarily a good one; Luffy has spent a great deal of time in the company of a sleeping Zoro, and never heard a name he did not know. Instinct, however, knows that Zoro sees that person when he sleeps.
The swordsman hesitates before replying, allowing his tongue time to wrap around the name. "Mihawk."
That much Luffy knows, has known for some time. There is someone else. "Not him. The other one."
Another hesitation, but Luffy is his captain and he is drunk. "Kuina."
The rubberman grins and gives Zoro a squeeze, and they continue in silence until Zoro, now uncomfortably close to sobriety, breaks it. "Don't you want to know who she is?" he demands.
"Only if you want to tell me," Luffy replies, instantly and completely honestly. He has learned enough from Zoro's tone and manner to feel assured.
Zoro almost doesn't want to, but does anyway, slowly telling Luffy his story. When he is finished, they are standing before Thousand Sunny and watching tiny waves lap against her sides.
Luffy turns to give Zoro a proper hug. Zoro is surprised, but not too surprised; he is tired, after all, and many strange things happen when he is tired. He hugs his captain back, allowing himself to just be for a moment, bathed in the light of the moon with the man who commands a third of his heart.