Shirahoshi

Princess Shirahoshi (しらほし姫), known as the Mermaid Princess (人魚姫), is a giant smelt-whiting mermaid and the youngest of King Neptune's children. She was first mentioned by Pappug, claiming he knew her and even promised to introduce her to the Straw Hat Pirates when they arrived at Fishman Island (though later this is revealed to be a lie).

Appearance
Shirahoshi is a giant-sized mermaid, and is described as "cute but huge" in her introduction. She has very long, flowing hair, with a fish-like hairgrip. When first encountered by Luffy, she wore a very revealing bikini with pearls on it and clam earrings. Her tail was split into two indicating she's at least thirty years old or older.

Personality
When she is first introduced, she proclaims she is not afraid of a potential attacker (which is what she believes Luffy is) but despite her size, she was rather frightened. She seems to have a very fragile personality, quickly breaking out into crying fits when Megalo disappears, when Luffy bounces on her breasts during their first meeting, and when Luffy says that he doesn't like her for being a crybaby. She is also very polite and adresses everyone with "-sama".

Because of her 10 years of solitude, she's rather curious as she asks Luffy about the sun, the animals and forests, and even asking him how he can eat so much for being so small. This prompts Luffy to scold her and have her burst into tears, which means that she is also bad at socializing, even more so when she was unsure of the idea of going out for a walk even though Luffy offered to protect her. She also does not seem to know about Devil Fruits as she called the powers of Vander Decken IX a curse.

Family
She has a pet shark named Megalo, whom she loves very much. When her beloved shark went missing, Shirahoshi got deeply upset and cried continuously.

Her father, King Neptune, and her brothers, Fukaboshi, Ryuuboshi, and Manboshi, all care for her well-being. They gathered an army to hunt down Vander Decken IX, a criminal who kept pestering Shirahoshi with love letters and threatening marriage proposals.

Enemies
Shirahoshi is terrified of Vander Decken IX for the threatening marriage proposals as well as many other unknown enemies who made attempts on her life.

History
According to Pappug, Vander Decken IX started sending letters to Princess Shirahoshi once a week and soon, those letters grew into packages. When he started sending threatening marriage proposals, she became so terrified of him that she couldn't ignore him anymore. This angered King Neptune, and his sons gathered an army to find Vander Decken IX.

Her pet shark, Megalo, had been missing for some time, having been captured and held in the Kraken's tentacles. Shirahoshi kept crying and crying until the shark returned to her, having been freed by Luffy after he defeated the Kraken.

When the Straw Hat Pirates were taken to the Ryugu Palace, Monkey D. Luffy wandered off on his own in pursuit of food that smelled good to him. Eventually, he stumbled upon her room, where Princess Shirahoshi was sleeping. It looked as though the room was attacked and seems like people were after her life. So it was built with the intention of keeping her safe from her attackers. Luffy was unknowingly walking across her stomach and fell between her breasts and bounce on them, thinking they were coral. When she woke up, she became greatly distressed. She thought that Luffy was one of the many attackers who have been targeting her life. She says she is not scared since she is the daughter of Neptune, but she starts crying to no end while calling for help.

As she continues crying, an axe comes flying into her room aiming for the princess. Luffy deflects the axe, saving Shirahoshi's life. When the guards come to Shirahoshi's room, the princess hides Luffy from them. Once the guards left, Shirahoshi speaks with Luffy. While Luffy is eating, Shirahoshi asks many questions about the outside world and wonders how Luffy can eat so much while poking him in the cheek. Luffy snaps back and Shirahoshi starts to cry saying that no one yells at her before. Luffy points that Shirahoshi is a big crybaby causing her to cry even more. Luffy then offers her a walk outside the castle with him being the bodyguard.

Trivia

 * The largest species of smelt-whitings is the King George whiting growing to a length of 72 cm and 4.8 kg in weight. Overall the smelt-whitings are a relatively small fish averaging at 20cm in length and 100g in weight; in contrast to Shirahoshi's size, which is on par with giants.
 * She is the second smelt-whiting revealed in One Piece, the first being Chew.
 * She is so far the largest mermaid seen in One Piece, and canon-like the largest female of the series.