He had always known that Sogeking was really Usopp. Chopper didn't though and Usopp himself didn't know he saw through his camouflage so Luffy pretended he didn't understand what it all meant. Pretended that he didn't realise Usopp had never left them. After all, why else would he come to help them, why else would he still care.

Besides, Luffy was the captain and he hadn't given Usopp his permission to quit the crew.

Usopp had been angry and hurt. Not that it excused what he had done, but Luffy had already forgiven him. Usopp was still with them and Robin wasn't really a traitor. They were all going to be alright.

Luffy gave the order and Usopp fired. And then the flag burned, but it was fine, after all it wasn't their flag and Robin was finally saying yes. Asking for their help, asking for Luffy's help.

So Luffy helped her. He and their nakama tore down all of CP9 and made them pay for hurting Robin and her kin. Merry came to save them and they escaped from the Buster Call. Robin was back, Merry was seen off and everybody was still alive, if not in the best condition.

Then Franky built Thousand Sunny for them and joined the crew too. Usopp was still nowhere to be seen and Luffy had started to worry. Because Usopp, though a coward was really brave and stubborn and might not continue to travel with them.

Without waiting the set sail and left Water 7 behind. They left Usopp behind.

But then Usopp was there and he was spouting nonsense, but Luffy wasn't worried anymore because he knew Usopp would soon apologise too. And that he did and Luffy took him back for they were nakama and all was already forgiven.

And things got better after that. Usopp, who had always been oh so insecure, was staring to come into himself, standing taller and prouder and stronger than he ever had before. And Luffy laughed and laughed and laughed and everything was alright, at least for a little while.