stupid
cat
After the battle with Aokiji, Usopp watched his friends easily beaten which scares him and makes him question his belonging in the crew. He was afraid that the crew had begun encountering stronger adversaries and as the Straw Hat's weakest fighter, he started to lose confidence in his abilities. A series of conflicts, particularly Luffy's choice to get a new ship, results in Usopp leaving the crew. After a battle with Luffy, where Usopp displayed an uncharacteristically vicious fighting style, he gained the now terminally-damaged Going Merry. Usopp was later detained by CP9.
After being freed from CP9 captivity by Sanji during the Sea train chase, Usopp announced that he has no business with the Straw Hat Pirates and disappears. Yet Usopp came back a minute later in a disguise. He was wearing a mask and a cape, using the alias "Sogeking" (sogeki means sniper, so Sogeking ("そげキング" Sogekingu) is a pun meaning "Sniper King", which is only pushed further when the World Gov. nicknames him "King of Snipers" Sogeking). According to his stories, Sogeking is from an island of snipers, Sharpshooter Island, that can be found in your heart, and Usopp has even written a theme song for this identity.
All of the Straw Hats (along with Galley-La, Franky Family, and CP9) immediately recognize his true identity except for Luffy and Chopper. For a long time, it was unknown if Usopp was going to rejoin the crew. His motive for concealing his identity is both so that he will not become a fugitive with an inflated bounty over his head, as attacking the Marine Base would cause this subsequent action and to keep his identity a secret towards the crew (even if it only worked on Chopper and Luffy)
He helped Franky and Sanji in order to rescue Nico Robin but they fail to do so. After being in the detached cart 4 and 3 with Sanji, they were found by the Franky Family and finally reunited with Luffy. When they confronted CP9, Sogeking unveiled a new weapon, a long staff with a more powerful slingshot attached called "Kabuto". Although he was the only Straw Hat unable to defeat a major CP9 agent, he did burn down the World Government flag on Luffy's command, signifying a declaration of war by the Straw Hats on the World Government. He also prevented the CP9 Chief Spandam from escaping with Nico Robin by sniping him from the Tower of Justice.
When first seen on his return to Water 7, Usopp was sitting on Oimo's shoulder, questioning whether or not to follow the giants to Elbaf. When Garp, Luffy's grandfather, paid a visit to his grandson, Usopp was peeking into the room along with Yokuzuna. Usopp is enjoying the large party in Water 7 along with everyone else, but as Sogeking. He now has a bounty of 30 million, however as his alter ego Sogeking, now known as "Sogeking, King of Snipers"". Interestingly, this is Usopp's second time appearing on a wanted poster with someone else's name on it, as he is also on Luffy's.
While he wanted to rejoin the Straw Hats, he would prefer Luffy to beg him to rejoin. However, Zoro has already advised Luffy not to do so until Usopp apologized. Usopp believes the crew would still come and fetch him, but after Zoro's speech the crew did not seek for him. During the bombardment of Garp on the Straw Hats, Usopp rushed back and saw the ship was leaving him behind. As Usopp tries to call to the Straw Hats, they ignored and continued fighting Garp. Usopp finally realized his wrong, shouting out his apology and taking back his words of leaving his companions. At that note, Luffy heard him and tearfully stretched his arm to get him back on board.
