Usopp walks in on Sanji crooning over a pan full of vegetables, his eyes closed as he reaches the highest note of the verse in perfect pitch and key. Usopp can't help but chuckle quietly to himself, but Sanji picks it up.
"Usopp." It comes out in a demanding way that catches the sniper off-guard, "What are you doing here?" Usopp realizes he's caught Sanji in an apparently vulnerable moment and shrugs to show it's no big deal, hoping not to upset the cook.
"I-It's the galley, Sanji, my workshop's in here." He saunters over to the platform on the floor and pats it as an example, "See?"
Sanji growls to himself and the room goes quiet. After a while, Usopp is the first to break the silence.
"You have a really nice voice." He says softly, trying not to upset Sanji any more than he already has, "I had no idea you could sing."
"Yeah well, it passes the time." Sanji's response is short and angry, dripping with contempt for the other boy. Usopp fidgets at the tone.
"You should sing more often. It's nice to listen to." Upon reviewing the sentence in his head, Usopp frowns. Nothing is coming out the way he wants it to.
"Thanks." Sanji's voice softens at the compliment. "You know. You're not so bad yourself." He turns to see Usopp's eyes widen slightly. Sanji nods, "Yeah I've heard you on watch. With your voice I didn't think you'd sing well."
"Th-Thanks." Usopp stutters while trying to figure out whether he was just insulted or not. He shakes his head, dismissing it.
"Maybe we should sing together sometime. Y'know…like a-"
"Duet?" Sanji finishes, "Yeah. That'd be nice."
That night, Usopp sat in the crow's nest, eyes towards the sky singing quietly to himself when he heard footsteps behind him.
"Mind if I join you?"
Sanji sat beside the sniper, blowing smoke towards the dark sky, "It's always nice to have a harmony."