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So, this was really not usually the sort of question a person asked Luffy. But Zoro was sleeping up in the crow's nest, and Robin was reading, and Sanji was on another ship entirely or maybe even another island, and no way in all five (six?) seas was Usopp asking Law.
And he really, really, desperately wanted to get away from the Barto Club people, because it was weird, being idolized when he hadn't spun the story to make himself so, and he didn't want to think about how he'd been willing to leave the Tontatta at Sugar's mercy either. He'd done too many things on Dressrosa that he wasn't proud of, that they made what little he was proud of seem insignificant. But then, that wasn't anything new.
"Um, Luffy..."
Luffy was sitting on top of the figurehead like always, back straight, eyes fixed to the horizon. The sight of him there, so comfortable in his own skin, was a kind of striking that Usopp wasn't used to anymore, after two years apart spent trying to convince himself he hadn't imagined everything so impossible about his captain.
Luffy locked his legs around the neck of the crudely-carved statue of him (which was still so weird, but hey, Luffy had been wanting a bronze statue for literal years so only fate and Nami had spared the Sunny weirder), tilted his head back a worrying amount, and said, "Usopp!"
"Um, I wanted to ask you something, actually," said Usopp, leaning on the railing.
Luffy tossed himself backwards off the figurehead, landing on his feet with the ease of repetition, and clambered onto the railing to sit next to where Usopp was standing. Then he tilted his head and eyed Usopp like he was waiting for something.
Which he was, Usopp realized a second later, he was waiting for Usopp to explain himself.
"So, I was wondering," he started, "what exactly is haki? I wasn't exactly stuck on an island with someone to explain it to me, like you guys." Sure, he’d gotten some impression of what it was and who could use it, but there hadn’t really been much time for explanations, yet. And the only way for him to survive out here, surrounded by monsters even worse than before, would be to know what made them tick and how to turn that against them, but Usopp didn't feel like explaining the logistics of being the weakest member of the crew to their strongest.
"I mean, it's haki," said Luffy, like that explained anything at all.
"Okay," said Usopp, bracing himself for a long conversation. Luffy wasn't good at explanations for the same reason he wasn't good at storytelling: he tended to assume that obviousness was universal and the interesting parts should always go first, all complemented by the attention span of a gnat. "So that was a bad question. What can haki do?"
Luffy's face cleared up considerably, and Usopp counted that a victory. "Well, there's three types." He held up a finger. "First there's Conqueror's haki, which you gotta have the right personality for so Rayleigh said it's pretty rare. You can knock people out with it! Basically, you like, push," Luffy mimed shoving something with both hands and nearly lost his balance on the railing, "out with your presence, which overwhelms people with weak wills until they faint."
"Oh," said Usopp. Scary, he thought privately. "I think I'm developing Don't-Go-Near-A-Conqueror-Disease."
Luffy laughed, shishishi, like Usopp wasn't entirely serious about this, like the terrors of the world were a source of delight, like Luffy always did. "It's no fun to get hit with, that's true! I made Rayleigh try it on me once to see. But don't worry, I won't hit you guys, you're my crew!"
Of course Luffy could use it. Usopp was not at all surprised, even if Luffy had made all that fuss about not wanting to conquer the seas back at Shakky’s bar. "So that's how you knocked out all those Fishmen?"
"Yep! Course you don't have to knock them out, you can also just make them run away, although that works better on animals and sea kings and stuff. And it's also good for making people listen to you!"
Usopp frowned, coming a conclusion he didn’t like the sound of. "Like, pay attention listen, or, like, obey listen?" If it was just the first, that would explain Luffy's non-existent stealth skills, but if it was the second... Usopp felt unease grow in his stomach. This was more alarming than the fainting thing, honestly.
"Hmm..." said Luffy, tilting his head. "Both?"
Of course Luffy would never use it like that, on his crew at least. But that absolutely didn't preclude someone else from trying the same thing. Conqueror’s haki was rare, restricted to that class of people who were legends or in the process of becoming such, but Luffy and Rayleigh were walking proof that it wasn't fictional. And with how legends populated the New World like regular people did elsewhere, the chances of getting Conqueror's haki'd were way too high for his liking, and he didn't want to betray anyone ever again, ever, and —
Usopp was catastrophizing, he realized, when he noticed Luffy staring down at him in concern and his breath coming fast.
He gripped the railing tightly, digging into it with his nails and forced himself, under Luffy’s gentle, unjudging gaze, to calm down until he could think of another question.
"So, what about the other two types of haki?" he asked finally, very proud of how confident he managed to sound.
Luffy stared at him for a few more seconds, leaving Usopp feeling like he had just been vivisected and then put back together with just those sharp eyes, and then he grinned and held up two fingers. "So the next two, anyone can learn! There's Armament haki," he gestured with a blackened fist that shimmered into eerie rainbows like oil as Usopp stared at it, "which lets you punch logias and also me, and also is basically like armor for your skin. And there's Observation haki, which — oh yeah, remember Skypiea?"
"How could I forget."
"Well, they had it there! It's that thing they called Mantra, that lets you sense stuff, like other people and their intentions and their emotions, and where they are and if they're gonna shoot you!"
Usopp frowned. "Sense like... see?" Something was nagging at him.
"Sorta?" said Luffy. "Rayleigh said it like, everyone's got an aura, so with Observation that's what you see, but for me it's sorta like everyone's all fuzzy with their emotions, even from all the way on the other side of the ship, or Zoro up in the crow's nest." He crossed his arms behind his head and sighed. "Man, I'm tired just thinking about Zoro."
Usopp looked up at him, balanced precariously on the railing, and wondered if he would have to jump overboard to save his captain's life again today. Especially with Zoro napping, although being asleep had never stopped their swordsman from diving after Luffy before.
Except... wait. Usopp hadn't gone up to the crow's nest since he had had watch last night. How had he known so certainly that Zoro was napping there? Auras, Luffy had said. Auras. Like the auras he'd been able to see on everyone ever since that desperate shot at Sugar?
For all his faults, Usopp was a storyteller. He was clever, he had a good memory, and when he wasn't crying in fear he could make connections from as far away as he could hit a target. Which was pretty far indeed, given that he could apparently use Observation Haki.
Pause. Don't overthink it. Get more information before you jump to conclusions. Was it really plausible for Usopp, the weakest among them, who had to rely on puffing up his fur to seem bigger than he was instead of any real strength, to be able to use something only the monster trio had been able to access?
The answer was no, of course. But that didn't account for how he could suddenly see auras.
"So," he said finally, when Luffy tapping his foot against the bars of the railing got too disruptive to his thought process, "with Observation haki, you could sense someone from across an island and snipe them from where you're standing, right?"
"Well," said Luffy, "I couldn't. You could, though." He frowned, leaning forward a little, and Usopp found himself compelled to look up and meet his eyes in all their endless darkness. "Can you?"
Usopp gulped. There was no lying to Luffy, not when he was focused on the totality of you like there was nothing else that mattered in the world. Usopp wouldn't, anyways, not to his captain, not about something like this, but having his defenses stripped away like that always left him feeling bare, and vulnerable.
"I... I think I can, actually. I think I got it, in Dressrosa. I was making this shot, and..." And he could open his big mouth, here, and tell Luffy about saving him and Law from Sugar, but suddenly he didn't want to. Suddenly he wanted to keep that moment close to his chest, hidden, so that Luffy would never know that he had almost been defeated and forgotten. Suddenly he could almost understand what Zoro had been thinking, that day at Thriller Bark. "And then, bam, I guess," he finished lamely, "I could suddenly see peoples' auras."
Unnoticed by others, I shoot down the enemy to protect my friends! In hindsight, Usopp couldn't believe that he'd actually meant what he'd said, then. And yet, somehow, keeping this from Luffy felt like the right thing to do. He was the crew's liar, on top of being their sniper, or beside it. Necessary misdirection could win you a fight, make the final twist in a story all the more satisfying, give the captain you owed everything to, your home and your courage and your life, some peace of mind in between worries.
Luffy clapped his hands together in delight. "Wow, Usopp!" he said, sparkles in his eyes and a rubber-wide grin across his face and all of him overflowing with that rapt pride Usopp could never quite bring himself to feel like he deserved. "Wow, Usopp, that's so cool! I have the coolest sniper ever!"
In one fluid, excited movement, he was off the railing and picking Usopp up to spin him around, laughing like the world had just given him something priceless. Usopp, too overwhelmed to do anything more than be limp in his captain's arms, let Luffy's joy flood over him until he was laughing too, and then they were both on the deck on their backs, trying and failing to catch their breath, and there was no moment more precious or pure in all the history of the world.