Hachi, also known as Hatchan, is an octopus fish-man who first appears as an antagonist before undergoing a significant transformation into a beloved ally and friend to the Straw Hat Pirates. His physical form is unique, featuring six arms and two legs, pink skin, and grey hair often styled into spikes. As an octopus fish-man, the undersides of his arms are lined with suction cups, allowing him to cling to walls. He also possesses the ability to shoot a cloud of ink from his mouth to blind opponents, much like a real octopus.

Originally a resident of Fish-Man Island, Hachi grew up alongside other future pirates like Arlong, Kuroobi, and Chew. In his youth, he shared a simple dream with them: to one day open a takoyaki stand. During this period, he also performed a significant act of kindness by saving a young Silvers Rayleigh from drowning after a shipwreck, forging a connection with the future vice-captain of the Roger Pirates that would last for decades. As he grew older, Hachi became a member of the Sun Pirates, the legendary crew founded by Fisher Tiger. During his time on this crew, he showed compassion to a former human slave named Koala, treating her with a gentleness that was not universally shared among his crewmates. He mourned deeply when Fisher Tiger died and eventually followed his childhood friend Arlong when the Sun Pirates disbanded, becoming an officer in the Arlong Pirates.

In his early appearances, Hachi serves as a loyal, if not particularly bright, enforcer for Arlong. He is directly responsible for a key tragedy in Nami's backstory, as it was he who discovered her cartography skills and brought her maps to Arlong's attention, leading to her forced servitude. His personality at this time is marked by a simple-minded loyalty and a fierce pride in his own strength, notably his six-sword style swordsmanship. Despite his goofy demeanor and a tendency to be easily tricked, he was a formidable fighter. He clashed with Roronoa Zoro at Arlong Park, where his six blades were shattered by Zoro's three, a defeat that illustrated the difference in their resolve and the "weight" they carried.

After the Arlong Pirates were defeated and captured by the Marines, Hachi was the sole member to escape from the prison ship. His journey back to Fish-Man Island became a period of profound change. During his travels, he befriended a mermaid named Keimi and a starfish named Pappug. Together, they saved a village of starving catfish, and the villagers, in gratitude, built him a floating shop. This allowed Hachi to finally achieve his childhood dream of running a takoyaki stand, called Takoyaki 8. When he reunites with the Straw Hat Pirates on the Sabaody Archipelago, he is a changed person. He is deeply ashamed of his past actions and genuinely seeks to atone for the suffering he caused, particularly to Nami. His personality has shifted to become openly kind, humble, and caring, acting as a protective older brother figure to Keimi and Pappug and going out of his way to assist his former enemies.

Hachi's role in the story becomes that of a guide and a bridge between worlds. He helps the Straw Hats navigate the complexities of the Sabaody Archipelago and introduces them to Rayleigh, who can coat their ship for the journey to Fish-Man Island. When he is shot by a World Noble for trying to protect his friends, the incident directly triggers the chain of events that forces the Straw Hats to separate for two years. Later, during the Fish-Man Island arc, he returns to his homeland and stands against the extremist ideology of Hody Jones. Hachi's own past, having lived through the hatred and oppression that Arlong spread, makes him a powerful voice for reconciliation. He works alongside Jinbe to stop Hody, hoping to break the cycle of hatred and prevent future generations from inheriting the same prejudices.

Hachi is a skilled fighter, possessing the immense physical strength common to fish-men. His primary combat style is Rokutoryu, or Six Sword Style, where he wields a massive katana in each of his six arms. These blades are incredibly heavy, each weighing over three hundred kilograms, yet he swings them with great speed and power. His suction cups allow him to grapple opponents and cling to surfaces, and his ink provides a tactical smokescreen. Despite his prowess, his straightforward and somewhat simple nature can be a weakness against more cunning strategists. Hachi's journey from a misguided villain to a remorseful individual seeking redemption makes him a compelling example of how characters in One Piece can grow and find new purpose, ultimately proving that friendship and forgiveness can overcome even a painful past.