One Piece has always had larger than life characters. Zoro can cut through just about everything with his swords. Whitebeard is capable of generating earthquakes with his fists. Luffy, by the time skip, is recognized as a major threat by the World Government. However, the Straw Hats' Usopp isn't quite on the same level as those around him. Usopp can't cut down platoons of marines like Zoro or stretch his body like Luffy. He's just a normal man, but that's not necessarily a bad thing.
Even his tragic backstory, by measurement of his friends', isn't especially tragic. Usopp grew up on a small island in the East Blue called Syrup Village with his mother and father. However, his father, Yassop, left his family when Usopp was still very young to become a pirate and join Shanks' crew. Not long after that, Usopp's sickly mother became bedridden. The long-nosed boy began telling tall tales not just to comfort his mom but also to give himself some clarity, telling his mother that his father was just over the horizon and that he'd back soon. After she died, Usopp continued to lie, claiming "pirates were attacking" to the villagers, which is what got him an unreliable reputation as a prankster. Usopp's childhood is the most human among the Straw Hats, which is fitting for the weakest member of the crew. With his mother incapacitated, all he could do was tell lies to comfort her, but more importantly, to himself, which is a big part of Usopp as a character.
Despite his father leaving him and his mother behind to become a pirate, Usopp has great respect for Yassop. He went to follow his dream, to take to a dangerous life of freedom and adventure and his son wants to do the same -- to become "brave warrior of the sea," as Usopp puts it. However, he's constantly struggling with his cowardice, scared of each every foe he and his friends go up against. Because the odds are so stacked against him, the moments he shines really stand out, asserting his underdog status.
From the first moment Usopp encounters Luffy, Zoro, and Nami, he uses lies to boost himself up. He claims he has 8,000 men behind him, but the trio sees right through this. When Zoro says that they're still prepared to fight, Usopp caves in and surrenders, but they quickly become friends with the sniper. Usopp still caves in this easily long into One Piece. An underrated example is during the Alabasta arc when Usopp teams up with Chopper to fight Miss Merry Christmas and Mr. 4. Usopp is terrified of dying after he realizes how strong the duo is, sending him running away crying. That being said, what makes Usopp such a stand out character is that there are times when he's able to stand tall and be brave despite the fear he's feeling.
When Miss Merry Christmas taunts Usopp by badmouthing Luffy, that's when Usopp gets serious. He's badly injured by the two bounty bunters' attacks but he gets back to his feet, still willing to fight, which gives us one of his best quotes in the series: "If you're a man, there are times when you have to fight. And that time is when your friend's dreams are laughed at." This is another example of what makes Usopp such an excellent underdog. When he does work up the courage to fight, it's usually never for himself, but for his friends.
During that fight in Alabasta, Usopp gets rammed straight through stone walls and hit in the face with a four-ton baseball bat. Usopp's covered in blood, he's trembling, and his nose is bent up like an old bendy straw. And yet he still pulls himself up off the ground, determined to beat the people who mocked his friend. In every arc, he's usually the one to get the most badly injured, and he often comes away by the skin of his teeth, too; either by tricking his opponent and using his lies against them, or watching them exploit their weaknesses. He's a scrappy fighter, but he can only take so much punishment before it truly starts to affect his confidence.
When the crew arrives at Water 7, they're bombarded by so many things going wrong so quickly. First, they find out their ship, the Going Merry, cannot be repaired, then they lose two-thirds of their money, Robin disappears, Usopp gets beaten by the Franky Family, and finally, Usopp argues against Luffy's decision to get a new ship, ending with them dueling for the Merry. It's a tragic moment, but one that speaks to how Usopp sees himself amongst his friends. Usopp knows he's the weakest member; how he always tries to repair the Merry is partially just to feel useful, but also how he identifies with the ship as well. The Going Merry takes as many beatings as Usopp but they always patch themselves up. But now, Luffy tells them this is where they part ways with the Going Merry, the ship that Usopp has been taking care of this whole time. In Usopp's mind, if it's so easy for the crew to leave behind the Merry, then it would be just as easy for them to leave him behind when he stops being useful.
After the duel with Luffy, Usopp ends up leaving the crew and you can feel the uncertainty that hangs in the air right up until Usopp returns to help the crew bring Robin back under the guise of Sniper King. This is another great example of how Usopp uses lies to bolster his confidence. He can't very well show his face to his former friends after everything he did, but he still cares about them enough to be here. Upon reuniting before the crew leaves Water 7, we get maybe one of Usopp's other best moments.
Zoro insists that Usopp can't come back to the crew unless he apologizes for going against the captain, Luffy. This is important because if they pretend that this transgression didn't happen they're essentially agreeing they don't respect Luffy as a captain, which means they aren't taking this way of life seriously, either. As the crew is departing, Usopp spouts every excuse he can make, trying to have his voice heard. It's only when he finally breaks down crying, admitting to the mistakes he made, that Luffy extends his arm out to him. It's a touching moment, as the sniper and captain are crying tears of joy, relieved to finally be reunited as nakama once again.
Usopp's had the most development out of all the Straw Hats, going from cowardly lying sniper to a genuine threat -- one that's gotten him a 200 million beli bounty by the time skip. He's sensible, but jokes around a lot, and underneath all that, is a man with the courage to stand up for his friends even when he's terrified. That's what makes Usopp a perfect underdog.