Koji Samehada, more widely known as Sea King(海王類, Kaiō-rui) is the first mate and a martial artist of the Dii Consentes Pirates with the alias Neptune and is a member of its Tribunal. Abandoned at birth by his parents in the deepest pits of the oceans due to his abnormal human appearance although he is a fishman, Samehada was found when a roaming Sea King heard him screaming. Most people believe Sea Kings to be savages, but very few know that they too sport intellect. Fearing for the boy's safety, the roaming Sea King took him in and presented Samehada to the King of Sea Kings, to which he adopted the boy. Teaching Samehada their ways, he quickly developed a fighting style called Sea King Kung-fu by observing the Sea Kings' movements.
The day Samehada turned 30, the King of Sea Kings escorted him to a secret cave where he told Samehada about their greatest prophecy. The King was to die later that day and Samehada would take his place as their kinds king, but would have to be transformed into a Sea King first to claim his birth right. The King warned Samehada that due to being in such close proximity to them for such a long time, his physiology would slowly change by every waning day until his transformation is completed.
Shocked by this notion, Samehada fled and left his adopted father to die. After reaching the shores of Fishman island for the first time since his birth, everyone saw him as an outcast and savage due to his ignorance about the civilized world. After being subdued by the local marine forces, he met Bakugou D. Erharde on their way to Impel Down. Telling Bakugou his current predicament, Erharde confirmed that a mysterious power at the All Blue would be able to halt this curses' spread, and that he would use all his might to help him if he joined his crew. Agreeing for he has nothing to lose, Samehada joined Bakugou and escaped from the marine escort unit together, utilizing his Sea King Kung-fu for the first time against other humanoids.
His power is a great asset to the Dii Consentes Pirates and displays it as one the main powerhouses of the crew. Samehada is slowly but surely learning of the civilized world, and is in constant awe because of it.