Hm, I've never tried UsoNa before so this was a first for me.
Luffy was childish yet adultlike, silly and goofy and idiotic all at the same time.
Zoro was real man, masculine and serious and fiercely loyal to the crew.
Sanji was a charmer, gentle with the ladies and absolutely nuts about them.
Franky was just plain weird, although Robin seemed to have already placed a claim on him and his crazy speedos.
Usopp was a liar, too proud and cocky and cowardly and afraid of everything but cockroaches, which Nami didn't understand. He was as childlike as Luffy and Chopper and Brook.
He was probably the weakest of the crew, not a close-range fighter like the Monster Trio or a crowd sweeper like Robin. His nose probably got in the way whenever he tried to sleep on his stomach.
He was the strongest of the crew, knowing when to stop fighting and when to stop joking around and get serious. He was as loyal as Zoro and as silly as Luffy and as suave as Sanji (that is, to say, not that much).
Nami sometimes didn't understand the sniper at all.
She especially didn't understand why he was the one. She didn't need to know why. She just knew.
Sometimes, after a long epic about how he escaped from an island of cannibal dragon-men on his glider made of palm leaves and branches, he'd quietly retreat to the kitchen to enjoy some leftover fish or to tinker with a little side project that had no purpose whatsoever, like a mechanical bird that chirped when its wings were pressed or a metal fish that swam around in a bowl of water until its screws fell apart and Usopp would go back to fixing it.
And then Nami would humor him just for the sake of messing with him (Usopp was nearly as cute as Chopper when he was flustered).
"Sogeking visited me last night."
"Wha- eh, I mean, he did? What?"
"Yup. He let me see under his mask."
"H-he did not! Nooo way! Stop lying!"
"I gotta say, Sogeking is a real hottie under that mask."
"... He is?"
"Mmhm."
And Nami would exit the room after pecking him on the tip of his long nose, leaving Usopp confusedly scratching his head with a perplexed expression on his face and the slightest blush.
Even if Usopp wasn't the strongest fighter around, he didn't need to be. He had his brains, and inventions, and quirky tales. He was Usopp, and it was as simple as that.
And Nami would finally understand just for a split second.