Usopp’s father was the sniper of the famous red-haired pirates, and since he was a pirate, he left his son when he was quite young. He was really proud of his father and his adventures; he also wanted to be like him someday.
Once Usopp’s mother got fatally ill, so to make her feel better, he used to scream that pirates are here every day and that his father was here to take them back through. When his mother passed away, he didn’t stop, making the villagers annoyed and angry. It also gained him the nickname of the liar.
In his village, there was also a massive mansion on top of a cliff. However, ever since the owner died, his daughter Kaya became sick because of depression, hearing that Usopp took it upon himself to make her better. Hence, every time he was free, he used to go to the mansion secretly, and he told her made-up stories to cheer her up and get her better.
Once actual pirates arrived on the shore, though it was nothing to fear since it was the straw hats and their captain Luffy was unlike regular pirates, he also knew Usopp’s dad when he was young. Hence, they become friends but not long after, they find out that the worker closest to Kaya was trying to kill her. Before that, he wanted to hypnotize her through one of his comrades and make her write a will, giving him all her properties.
Luffy fights him for Usopp and wins, then Kaya gifts them their very own ship called Going Merry.
Usopp also becomes their sniper and the fourth official member of the straw hat pirates.