AN: do not own one piece and so do not know all the specifics of Haki so sections of this may be going against Canon, but I think all of this is technically possible.

There are over 30 thousand stories for one piece, I haven't read them all, if this has been done before let me know so I can take it down.

Smell

'Hey, Luffy, how do you and Chopper not know that Sogeking is Usopp? It's a rather obvious trick.' Nami asked in that all to innocent voice of someone that was frustrated with something that was, really, none of their business.

'What are you talking about, Nami? It's impossible for Usopp to be Sogeking.' A few meters away Usopp blushed as he listened in carefully, while pretending to be making something. The rest of the crew (bar Chopper who was in his room) also carefully pricked their ears from various places around the ship. It was something that had been confusing them for a while now, how those two could so oblivious when they were normally the best at finding lost crew members.

'Luffy,' Nami gave a sigh. 'Why is it impossible? I mean, they both have the same long nose, the same voice and could practically be the same person, how do you and chopper know that they are different people?'

Luffy looked at Nami as if it was completely obvious, 'they smell different.' And with that, he walked away.

The rest of the crew looked at each other in surprise and Usopp tried to subtly test if he really did smell that bad, to the point where it was noticeable to Luffy.

'Huh,' was the only reply to the gathering silence, it was really the only response appropriate.

Haki

It was during the fight in Dressrosa that both Sanji and Zoro paused, only for a second as their own battles continued whether they were ready or not, at the strange phenomenon they had just witnessed.

The presence that they knew had been there, that was as familiar to them as their ship or the faces of their Nakama (and wasn't this just another kind of face?) vanished, and was replaced by something else entirely even if only for a few short seconds. Whether they noticed it or not, both had been using Haki to keep track of those they cared about during the fight, and Usopp dropping out of the picture entirely was not something they could overlook.

Except he didn't vanish, rather, it was like, for a few seconds, his aura had grown and warped into something unrecognisable as the person that was the crew's resident liar. It didn't last long, but it was enough to keep them up long into the night when they should have been collapsed from exhaustion, and remembering a conversation that had occurred soon after they had picked up Brook.

Luffy had said that Usopp couldn't be Sogeking because he smelled different, apparently that wasn't the only thing to change when their sniper put on the mask, whether it was physically or not.

Mask

Usopp knew he was Sogeking, there was no way for him not to know, and yet even he had to admit sometimes that the change that came over him when he shifted that flimsy bit of wood over his face was rather drastic. The complete change from the lying coward of the village to a person who could stand beside the monsters in his crew with pride and confidence. Sometimes it felt like he was never more himself than when he was hiding who he was, other times it felt like someone completely foreign had settled inside his skin. It was exhilarating and petrifying, a comfort and yet deeply terrifying. The little voice in the back of his mind that he recognised as his, even as it spoke words he would never, could never, say.

There was a barrier, and yet as the years passed and his bounty grew he found that barrier had begun to crumple. The voice got quieter as he found the words himself, and he grew into and out of the mask that had grown into something bigger than he thought possible when it was nothing more than a way to escape embarrassment.

Little by little the two bleed into each other, and he lived for the day when the barrier completely ceased to existed and he could become the person that had looked into his own past to give advice to the struggling boy as he learned to grow.