Chimney

Chimney is the hyperactive granddaughter of Sea Train conductor Kokoro, and owner of the rabbit, Gonbe (a rabbit that thinks it's a cat). She takes a liking to the Straw Hat Pirates instantly when the crew meets her outside of Water Seven. Franky called her "shrimp". She also tends to notice things that others cannot, such as when she spotted Luffy wedged between two buildings (which is where he landed after Rob Lucci knocked him out of Galley-La Corporation Headquarters) or how she managed to find secret passageways in the Tower of Justice at Enies Lobby. She, along with Gonbe, later sneak aboard the prototype Sea Train, Rocketman, and accompany the Straw Hats, Franky Family, and Galley-La foremen to Enies Lobby. And later helped Luffy find the secret passageway that Spandam, Lucci and Robin took on the way to the Gate of Justice.

As Kokoro is revealed to be a mermaid, it is possible that Chimney, being her granddaughter, may have latent mermaid abilities of her own. Although Chimney doesn't seem to have any mermaid characteristics, she was able to save Luffy, who is 3 times her size, from drowning off screen while resisting what must have been an overwhelming current from the several whirlpools in the sea in front of the Gate of Justice.

Currently, she is in Water Seven. In the Volume 45 SBS, Oda revealed that Chimney is indeed 1/4 mermaid, and is thus an abnormally good swimmer.

(Source: Wikipedia)