Kokoro

Kokoro (ココロ) is the main conductor of the Sea Train, Puffing Tom, at the station just outside of Water 7 and a mermaid. She has a granddaughter named Chimney, who accompanies her everywhere.

Appearance
Her facial features make it look as if she is perpetually smiling regardless of her actual emotions. Her body shape, along with her strange smile, makes her look like a frog. (She is, in fact, an icefish mermaid, though she resembles more of a dugong in terms of appearance and shape).

Personality
Kokoro is a heavy drinker, and speaks with a mild speech impediment that gives her dialogue a distinctive ring. While she does have a temper she tends to keep it under control and seems to remain happy no matter what events unfold remaining strangely relaxed and calm.

She takes a liking to Luffy and eventually refers to him as the Pirate King. She is very curious about him because the events at Enies Lobby became very interesting. She came to the conclusion if Tom were still alive he would have helped him just as he had helped Gol D. Roger.

Abilities and Powers
Although she's not a fighter, she has one named attack, called Mermaid Kick, with which she kicks an opponent with her fishtail-like leg with enough strength to send a marine flying far away. She is a fast swimmer, like all of the race.

Early Life
She left Fishman Island to support Tom and met her human husband in Water 7. Together they had a son who was half human/half merfolk.

Tom's Secretary
Kokoro became the "lovely" secretary of Tom, and a matronly figure to his two apprentices, Iceburg and Franky.

When Tom was taken to trial in 1500 AOS, Kokoro was watching the whole thing. After Tom was given a ten year sentence to build the Sea Train (lengthened to fourteen years later on), Tom laughed about being given a chance, but Kokoro had been scared by Tom's brush with death, and did not appreciate him laughing about it.

At some point during the next fourteen years, Kokoro visited Enies Lobby with Tom, and memorized some of its layout for future reference.

When the sentence was nearly done, the judge, impressed with Tom's work, intended to pardon him. However, Spandam's CP5 interrupted his verdict by attacking the judiciary ship with the Battle Franky ships, thereby incriminating Tom's Workers. It was Kokoro who stopped Franky from making matters worse by fighting back.

When Tom was captured by Spandam and his agents, Iceburg, Franky, Kokoro, and Yokozuna were brought to trial. To save Iceburg and Franky Tom sacrificed himself by saying he was proud to help Gol D. Roger, and wounded Spandam. After Tom was tranquilized, the Judge decided to take only Tom and free his workers. Kokoro hugged Franky and Iceburg as she explained that convicts never come back from Enies Lobby. Furious, Franky attacked Spandam with a rifle butt stolen from a marine and permanently disfigured his face. After Franky was run over by the Sea Train, Kokoro announced that Franky was dead and she then became Iceburg's drinking buddy.

After this, she took up a new job as a stationmaster of the Sea Train's Blue Station. During Franky's eight year hiatus, Kokoro grew with age, becoming a heavy drinker, bitter about losing Tom and angry towards the World Government. Later, her son left Chimney and Gonbe under her care.

Water 7 Arc
Eight years later, when the Straw Hat Pirates arrived at the Sea Train station, they met Chimney, Gonbe, and a drunken Kokoro. Kokoro told them about the Sea Trains and how they work, then offered them the Sea Train route to Pucci, San Paldo, or St. Popler but Nami told her that they must only follow the Log Pose. Kokoro told them that Iceburg should be able to help them repair their ship, and wrote a note to this effect, accompanied by a poorly-scrawled map intended as a guide to Water 7. She later appeared at a bar where Blueno worked, Franky was also there. During a conversion with Franky, Kokoro talked about an organization that was supposed to be secret, CP9. Later that night, Kokoro, Chimney, and Gonbe walked through the town as the Aqua Laguna approached to the island.

Enies Lobby Arc
After CP9 took Nico Robin into custody and the Straw Hats resolved to rescue her, Kokoro volunteered to help the pirates. They were joined by Paulie, Peepy Lulu, Tilestone, and the Franky Family. She piloted the sea train, Rocketman, onto the train tracks. As the train headed toward a giant tidal wave, Kokoro witnessed Luffy and Zoro create a giant hole in the wave for the sea train to pass through. When they finally reached Enies Lobby, Luffy went ahead on his own and Kokoro commented on how impatient Luffy was. As the Franky Family and the Galley-La foremen were battling Oimo after defeating Kashii, Kokoro assisted them by crashing the Rocketman into the giant, knocking him out. When the train stops in the Enies Lobby mainland, Kokoro became an onlooker of the battle. She later assisted the Straw Hats in entering the Tower of Justice with the Rocketman. After Chimney discovered the secret underground passage, she led Kokoro and the pirates to it after Luffy ran after Lucci and Robin. When Lucci flooded the passageway during his battle with Luffy, Kokoro revealed her mermaid form and carried the group through the tunnel, out into the sea, and on to the government ship that Franky and Robin had taken control of. Kokoro, Chimney, and Gonbe watched over an immobile Chopper as the other Straw Hats battle the Marines. When the Marines learned that the pirates were planning to use the ship to escape, they had their warships destroy it. Luckily, Sanji saved Kokoro, Chimney, Gonbe, and Chopper by carrying them off the ship in the nick of time. When the Marines had the group cornered, they hear the Going Merry. They jumped into the sea, boarded the Going Merry, and ultimately escaping Enies Lobby.

After sailing far away from Enies Lobby, the group meet up with Iceburg and the Galley-La workers. The Going Merry breaks down and Kokoro attended its Viking funeral along with the rest of the group.

Post Enies Lobby Arc
Kokoro and the group then returned to Water 7 and stayed with the Strawhats in the Galley-La headquarters. When Vice-Admiral Garp arrived to see Luffy, Kokoro was shocked to learn that Garp is Luffy's grandfather. Kokoro later swam in the Galley-La pool as Nami eavesdropped on Luffy and Coby's conversation. She then attended the Straw Hats' big party. After the Straw Hats' Log Pose pointed to Fishman Island, Kokoro explained to the pirates that they will have to cross the Florian Triangle before they can reach Fishman Island. When Sanji started to lose his excitement of seeing mermaids after seeing Kokoro as a mermaid, she pointed out to the chef that there are younger mermaids. After Franky, Iceburg, and the Galley-La workers finished constructing the Thousand Sunny, Kokoro, Chimney, and Gonbe then see the Straw Hats set sail on their new adventure.

Early One Piece
She was originally planned to be male.

Translations and Dub Issues
"Kokoro" translates from Japanese as heart, in the emotional sense (such as "follow your heart", not the actual bodily organ), or emotion.

Trivia

 * Like some of the other characters in One Piece, Kokoro has a unique laugh, she starts it with a "Nga" (i.e. Ngagagaga!).
 * Her appearance led to a brief joke when the Straw Hats met Keimi, a younger, much more attractive mermaid. When many of the male Straw Hat crew members see Keimi, they seem to attempt to repress their memories of Kokoro being a mermaid, pretending that Keimi is actually the first mermaid they ever met. Luffy also believes this at first, not having been present when Kokoro revealed herself as a mermaid at Enies Lobby, and considered the idea of Kokoro being "disgusting", much to Nami's fury.
 * Despite being a mermaid, she is seen with scaleless ankles exposed early on in the manga. While this could be because she was still wearing stockings or socks, the anime showed her with flesh colored ankles, indicating her ankles were normal skin and not her mermaid tail.