Neptune is a coelacanth merman who serves as the king of the Ryugu Kingdom on Fish-Man Island and the patriarch of the Neptune family. He is the father of the three princes Fukaboshi, Ryuboshi, and Manboshi, as well as the mermaid princess Shirahoshi, and was the husband of the late Queen Otohime. Born and raised in the underwater kingdom, Neptune inherited the throne and became known by the epithets God of the Sea and the Great Knight of the Sea, reflecting both his immense physical presence and his reputation as a formidable warrior. In his younger years, he was regarded as one of the strongest fighters beneath the waves, having sailed alongside the legendary fish-man liberator Fisher Tiger and earned the respect and friendship of powerful figures such as Edward Newgate and Gol D. Roger. Despite his towering size and formidable strength, Neptune possesses a generally easy-going and somewhat carefree personality, often acting on his own whims to the occasional frustration of his ministers. He is deeply devoted to his family, especially his daughter Shirahoshi, whom he dotes on and seeks to protect from the dangers of the world. His late wife Otohime championed the ideal of peaceful coexistence between fish-men and humans, and Neptune consistently supported her vision even as he struggled with the weight of protecting his people from discrimination and external threats. After Otohime's death, he carried forward her hopes while maintaining a cautious stance toward the surface world. Neptune's role in the story centers on the Fish-Man Island arc, where he initially extends hospitality to the Straw Hat Pirates after they save Shirahoshi's pet shark Megalo, but later attempts to capture them upon hearing a prophecy from Madame Shyarly that they will bring disaster to the island. This attempt backfires, and he ends up tied up by the pirates. During the coup led by Hody Jones and the New Fish-Man Pirates, Neptune is imprisoned and nearly executed at Gyoncorde Plaza, but he remains resolute in his determination to protect his family and people. The Straw Hat Pirates, led by Monkey D. Luffy, rescue him and defeat Hody Jones, restoring peace to Fish-Man Island. Neptune's gratitude toward Luffy and his acceptance of the Straw Hats marks a significant turning point for the kingdom's future, as he chooses to embrace cooperation between species rather than isolation. He later attends the Reverie, the world government's summit of monarchs, where he is escorted by Monkey D. Garp, a long-time acquaintance. Among Neptune's notable abilities are his natural strength as a giant fish-man, mastery of the Merman Combat fighting style, and a technique called Mermaid Jujutsu Super Stream, which allows him to grasp and release ocean currents to create powerful whirlpools capable of overwhelming large groups of enemies. He wields a massive trident in battle and commands a loyal pet whale named Hoe, which he often rides when traveling outside the palace. His authority and strategic experience enable him to command respect from his subjects and maintain order throughout the Ryugu Kingdom, even in times of crisis.