King Neptune is a coelacanth merman and the king of the Ryugu Kingdom on Fishman Island. He is the father of Fukaboshi, Ryuboshi, Manboshi, and Shirahoshi, and the husband of the late Queen Otohime. Neptune is a giant-sized and muscular coelacanth merman. He has a red nose, small eyes, and long thick orange curly hair and beard. Both his chest and arms are covered with hair. He is bare-chested and has matching black tattoos on both arms, which looks like a spiral with wavy protrusion designs around it. As king, he wears a gold crown that resembles a tulip with jewels embedded, a golden bracelet on each wrist, and carries a gold trident with an elaborate design.
Neptune seems to be very blunt, as he simply invited the "Straw Hat humans" to the Ryugu Palace, instead of being more cordial. He also tends to shout his words out for no reason, including a habit of shouting his own name at random times, being followed by his whale shouting "Hoe". Neptune sometimes mispronounces people's names, as he refers to Zoro as "Zori". He appears to be easy-going, absent minded, and carefree as he was scolded by his ministers for leaving the castle on a whim on more than one occasion. He has a tendency to finish his sentences with the word "jamon". However, he can be serious when the situation demands it, as he intended to capture the Straw Hat Pirates when they were wrongly accused of kidnapping mermaids, though Neptune did not think it was fair to arrest them based on a prediction made by Madam Shyarly, and merely wanted to capture the crew rather than arrest them for a precaution. In addition to being fair, while in his youth he would kill others for revenge, in the present he has shown to be very merciful, as he allowed the human slaves of the New Fishman Pirates to leave and let the lower fishman subordinates to do penal work under the watch of the army.
He dotes on his only daughter Shirahoshi, and worries about her immensely. He provided several extreme but necessary security measures to protect her from Vander Decken IX, and even attempted to beg the Straw Hats to rush to her aid when he thought that his daughter is in danger. Neptune apparently acknowledges the value of sacrifice, as he decided that he would rather risk his daughter's safety in the hands of strangers than in Decken's captivity. He gets quite upset when he hears that Shirahoshi is upset and gets very emotional when she is in danger as shown when he burst into tears when Shirahoshi was supposedly kidnapped and when Decken went after her. Neptune is also partially perceptive as he correctly guessed that Luffy snuck Shirahoshi out of Hard-Shell Tower by stuffing her inside Megalo, even though everyone else thought this was a bizarre idea. However, he can jump to conclusions when he stopped suspecting Luffy only after Vander Decken IX appeared in the palace and quickly accused Decken of kidnapping Shirahoshi. He stated when his wife died, that even if the person who killed his wife was unarmed he would have torn them apart. However, moments later he noted Otohime would not approve of a revenge attack and though he would only have listened to those words because it came from her, he still could not understand why he could not bring himself to tear that person apart. He did not understand how the events led to the people of Fishmen Island and humans hating each other. He however, wanted to end the cycle of hate between humans, he noted due to his past as a soldier he could not be a candidate for causing such a thing as he would only cause the opposite of his intentions to happen. He understood how the circle continued though, that children who witness darkness only continue the dark path that has caused their problems, but time could heal those scars and pain. Neptune is well aware of the Void Century and Poneglyphs, as well as the dangerous potential they pose to the world. Despite doting on his daughter, he knew she is the incarnation of the deadly ancient weapon, Poseidon, and fears of her power having fully awakened. He was also shocked and seemingly slightly frightened that Nico Robin is able to read about Joy Boy in the Sea Forest's Poneglyph, but trusted her enough to tell her about the history in detail when she asked. Like many other characters in the series, he appears to have a unique laugh: Hoh hoh hoh hoh.