When Nami got sick, it was Luffy, Sanji, and Usopp that all piped up saying they had never been sick before. Luffy we figure his constitution from his fruit, before that maybe just strong genes or something. Sanji, his genetics, since even if the other parts didn't showup, he was still super durable and resilient over most humans.
So what about Usopp? What non-human life changing secret do we find there?
So what about Usopp? What non-human life changing secret do we find there?