Neptune, sovereign of Fish-Man Island, rose to power following his father’s demise, fulfilling a prophecy foretold by the mermaid seer Shyarly. His union with Queen Otohime, a passionate advocate for unity between fishmen and humans, yielded four heirs: Fukaboshi, Ryuboshi, Manboshi, and Shirahoshi. Otohime’s tragic assassination by Vander Decken IX left him grief-stricken, yet he steadfastly upheld her vision of peace. When the Straw Hat Pirates arrived on Fish-Man Island, Neptune summoned them to his palace to gauge their motives. His daughter Shirahoshi, cursed by Vander Decken’s relentless pursuit, remained sequestered within the palace walls for protection. Amid Hody Jones’ violent uprising, Neptune was imprisoned and sentenced to public execution at Gyoncorde Plaza, only to be freed by the Straw Hats’ decisive intervention. As Fish-Man Island’s delegate to the Levely summit, Neptune sought to formalize the kingdom’s alliance with Monkey D. Luffy, leveraging Jinbe’s pledge to the Straw Hats. There, he thwarted Celestial Dragon St. Charlos’ bid to enslave Shirahoshi, aided by St. Mjosgard’s unexpected mediation—a pivotal moment validating Otohime’s faith in mutual understanding. Neptune’s reign intertwined with ancient legacies when Shirahoshi’s identity as Poseidon surfaced, granting her dominion over Sea Kings. He guarded the kingdom’s ties to Joy Boy, a forgotten figure from the Void Century who had pledged to utilize the ark Noah for Fish-Man Island’s salvation. Beyond the throne, Neptune contended with mounting challenges: appeasing Big Mom’s tribute demands, heeding Dr. Vegapunk’s warnings of oceanic upheaval, and steering his people toward coexistence while preserving their fragile hopes amid a turbulent world.