Luffy and Uta have met each other in their childhood, when Luffy was 6 and Uta 8, due to Shanks arriving in Foosha village and making it his main operation base. While at the start Luffy and Uta didn't hit off in the best of ways, with her finding him annoying and him being irritated by her mocking of him, the two eventually warmed up to each other, thanks to their constant challenges to prove who was stronger and basically spending their entire time together.At some point, shortly after meeting, Uta revealed to Luffy of being the Red-Haired crew's musician and sang for him. Luffy admitted her ability, even if his singing wasn't appreciated back by her. Her singing however, kept inspiring him to sing more, and also thanks to Shank's explanations about the importance of a musician in a crew, Luffy grew very passionate about music.In Oda's notes and the approved anime episodes on their childhood, it's seen how they often bickering but defined as two who "get along very well". Uta often cheats on their competitions, mocks Luffy for being two years younger and tries to act bossy, lamenting that he is often overhyped, but eventually always lets him convince her easily to tag along and join on his games and crazy adventures, showing she is secretly as invested as him. Sometimes, when not too bothered by just relaxing and listening to Uta's discourses about imagining adventures only in the head, Luffy would lay down with her on the grass and listen to her singing, until falling asleep (as shown in Oda's notes)They bonded over their challenges, between chicken races, knives throwing, arm wrestling, meddling with mountain bandits etc...
