Usopp the Strongest? Leave the Negatives to Me

Luffy continues on to take on Moria while Chopper and Robin remain behind to take on Hogback and his loyal zombie servants. After Oars runs off with a new pirate hat from his destruction, the remainder of the crew end up in Perona's room where she uses her negative powers on them. However, Usopp reveals that he's immune to her attacks because he's already got a negative personality, staying behind to take on the ghost princess while the others continue towards Absalom and Ryuuma.