Karoo, also spelled "Carue," is Vivi's pet spot-billed duck and one of the fastest modes of transportation available in Arabasta. He leads the Super Spot-Billed Duck Squad in Arabasta.
Karoo is incredibly loyal to Vivi and her friends, the Straw Hat Pirates. His behavior is quite unique; sometimes he follows instructions precisely, like offering his hand, while other times he does the opposite, such as drinking all his water despite Vivi telling him to conserve it in the desert.
During Vivi's time with the Straw Hat Pirates, Karoo mainly serves as comic relief. However, he occasionally provides valuable information to Chopper, helps Luffy and Usopp defeat Baroque Works agents in Little Garden, and saves Vivi from Mr. 2 Bon Kurei in Arabasta. He is the focus of several jokes in the Arabasta arc, including being used as fishing bait by Luffy and Usopp, much to Vivi's anger, and being mistaken for an ostrich by Sanji.
(Source: One Piece Wiki)
