Captain Kuro Arc

After Nami forms a partnership with the two as their navigator (though she is quick to point out that she is not a crewmate), they reach an island and search for a ship to use. There, they encounter a liar named Usopp, who too has ties to Shanks' pirate crew. When a devious plot is revealed, Usopp's courage is put to the test and the Straw Hats have to fend off Captain Kuro from destroying Usopps home town..

This is the arc were Usopp joins the crew.