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 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 Gold 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 twelve year sentence to build the Sea Train, 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 twelve years, Kokoro visited Enies Lobby with Tom, and memorizes 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 form 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 Blue Station. During 8 years, 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
8 Years later, the Straw Hat Pirates arrived at the Sea Train station, they met Chimney, Gonbe and a drunken Kokoro. Kokoro told them about the Sea Train 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 the ship, and wrote a note to this effect, accompanied by a poorly-scrawled map intended as a guide to Water 7.

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. Nagagagaga!).
 * Her appearance led to a brief joke when the Straw Hats meet 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.
 * Despite being a mermaid, she is seen with scale-less ankles exposed early on in the manga. While this could because she was still wearing stockings or socks, the anime showed her with flesh coloured ankles, indicating her ankles were normal skin and not her mermaid tail.