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, approximately the size of her pet Megalo, but still smaller than her father, and is described as "cute but huge" in her introduction. When first encountered by Luffy, she wore a very revealing bikini with pearls on it and clam earrings. She has very long, flowing hair, with a fish-like hairgrip. Her breasts are large and much of her upper body is exposed. According to Luffy, her breasts are like pudding.

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 easily 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. Nevertheless, she willing hides Luffy from the guards after he saves her from being struck by an axe thrown by Vander Decken IX and also thanks him for rescuing Megalo. She is very polite and addresses everyone with "-sama". She is also very considerate of her family and kingdom, saying that leaving Hard-Shell Tower even if she wants to would be a selfish thing for her to do.

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 is somewhat spoiled, as she claimed that nobody ever yelled at her before, and cried at the fact that Luffy would be the first to do so. She also does not seem to know about Devil Fruits as she called the powers of Vander Decken IX a curse. Much like the citizens of Kuja, she has very limited knowledge of the world outside her tower, after being confined in there for a decade, and like the Kuja, she takes Luffy to be the norm for humans.

Family
She has a pet shark named Megalo, whom she loves very much. Megalo is the only friend she can talk to while being locked up in Hard-Shell Tower. 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.

Monkey D. Luffy
At first, Shirahoshi thought that Luffy is an assassin aiming for her life and feared him. However, when he blocked a flying axe that was aimed at her and figured out that he was the one who saved Megalo, she warmed up to him. She even hid him from her guards when they were searching for Luffy after he was wrongly accused of kidnapping mermaids and after Madame Shirley claimed envisioning Luffy destroying Fishman Island. Luffy, on the other hand, thought of Shirahoshi as a big crybaby who talks too much and yelled at her for it, which led her to cry. Shirahoshi tearfully stated that Luffy is the first person to yell at her.

Enemies
Shirahoshi is terrified of Vander Decken IX for the threatening marriage proposals. Decken is completely obsessed with the Mermaid Princess and will do anything to get her and plans to kill her if she refuses, to ensure that nobody else would have her.

Past
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. Because of Decken's ability to hit his targets from anywhere, Shirahoshi had been locked in Hard-Shell Tower for ten years for her safety.

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.

Fishman Island Arc
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 thrown by Vander Decken IX 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. She tells the guards that the noise they heard coming from her room was her having a bad dream. The Minister of the Right explains the situation with the Straw Hats. Once the guards leave, Shirahoshi speaks with Luffy. While Luffy is eating, Shirahoshi asks him 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 had ever yelled at her before. Luffy points out that Shirahoshi is a big crybaby causing her to cry even more. Luffy then offers to take a walk with her outside the castle with him being the bodyguard.

Luffy asks Shirahoshi about where she wants to go. The princess says that she wants to go to the Sea Forest. When she starts crying again, Luffy starts to refer to her as "weakling". Knowing that Shirahoshi's size will draw attention, Luffy comes up with a plan. As Brook and the Minister of the Right arrive at her room, Megalo bursts through the doors with Shirahoshi hiding in his mouth.

As Shirahoshi, Luffy, and Megalo are leaving, the Mermaid Princess promises that she will be back by dinner. She then tells Luffy how exciting it feels to be outside again, saying it was an adventure. Once they are outside of the palace, Shirahoshi tells Luffy that in the Sea Forest, there is a grave that she wants to visit.

Anime and Manga Influences

 * Shirahoshi's character has several references to fairy-tales:
 * Some stories have kings with multiple sons and a single daughter. The only sister is usually the one who protects/saves her brothers, such as in "Ancilotto, King of Provino" and "The Wild Swans". "Childe Rowland" is one of the few exceptions.
 * In "The Frog Prince", a spoiled princess was disgusted by a frog but reluctantly befriends him after he retrieves her golden ball for her. Shirahoshi was initially afraid of Luffy but quickly befriends him when he saves her, she then learns that he's the one who rescued her pet shark.
 * 'The Maiden in the Tower' is type 310 of the Aarne–Thompson system for classifying folktales. One of the the most famous is "Rapunzel". King Neptune locked his daughter away to protect her; why a maiden is locked up varies, but it is usually for negatively selfish reasons from her caregiver (such as jealousy).
 * Neptune protecting Shirahoshi from an unwanted suitor is a reversal of most type 510B stories, 'The Persecuted Heroine', where the father either wants to marry his own daughter or marry her off to someone she doesn't want. In "The Bear", the king was overprotective of his daughter and locked her up (not necessarily in a tower) to keep her from any harm (and by extension, any suitors at all).
 * "The Little Mermaid" is the youngest of her siblings (except Shirahoshi has three brothers instead of six sisters) and is very curious about the outside world. Also, Shirahoshi being tormented by an unrequited love is similar to the original version by Hans Christian Andersen, except Shirahoshi is the receiver while the story's mermaid is the giver (to a prince who later marries another woman).

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.
 * Luffy's treatment of Shirahoshi is similar to how Ace treated Luffy during their childhoods, and how Luffy treated Coby when they first met.