"Hundredth Dan" Hack is a Japanese soldierfish fishman and member of the Revolutionary Army. He went to Dressrosa to investigate the arms trafficking there and competed at the Corrida Colosseum for the Mera Mera no Mi. Hack is a fishman with yellow skin that has brown spots on the upper part of his head, along with scales under his eyelids, sharp teeth, and gills on the sides of his neck. The fishman has a muscular stature, with long light blue flowing hair and a thick blue mustache. Similar to Kuroobi, he wears an open gi with a black belt. When he was transformed into a toy, he appeared to be shaped like a dog.
Hack is a calm and proud person, and follows a dream where a path of friendship may be formed by fighting. For that reason, he will continue fighting. He finds fighting battles to be honorable and thus, dislikes those who show a lack of respect toward a fight such as Bartolomeo. He is very honorable and dislikes attacking an opponent from the back, but he will do it if said opponent is not worthy of his respect, as he did to Bartolomeo, who was urinating on the side of the fighting ring and was, in Hack's opinion, not taking the battle seriously. As a fishman, he is at least ten times stronger than an average human, and twice that in the water. However, as he is taking part in the Corrida Colosseum fights, a location in the New World, as well as from the demonstration of a high-level Fishman Karate attack, it is implied that Hack's much stronger than that. He was able to stop an assault from a large gladiator by grabbing his opponent's spiked club, with the man commenting that he was unable to move his weapon in Hack's grip. At an unknown time in the past, he became friends with Jinbe of the Royal Shichibukai . He also became a member of the Revolutionary Army where he trained many orphans over the years, including Sabo. By the time Sabo was 13, who was so strong even for his age, Hack had nothing left to teach him. A year later, Hack and Sabo became acquainted with Koala. Over the years, the trio were sent on the same missions together. After the Whitebeard War, Sabo had a mental breakdown after recovering his lost memories. When Sabo awoke from his three-day coma, Hack expressed his concerns.
After Block B's battle royale had started, one of the fighters taunted Hack for being a fishman, asking why a "freak" like him had entered a human tournament. Eyes closed, the fishman mused over how pitiful the man's statements were, remarking that they are nothing but the words of a puppet, and that if he wants to talk, he should at least do it with his fists. Enraged, Hack's opponent rushes forward and attacks him with a giant spiked club, which the fishman catches with one hand. As the man wonders why he can't move, Hack defeats him with Fishman Karate, sending him flying away. Hack then announces his desire to continue pursuing his dream of forming friendships through fighting, in the same spirit as his friend Jinbe. As he continued to defeat opponent after opponent with his Fishman Karate, Hack gained popularity from both the crowd and Gatz, who heartily cheered him on. When Bartolomeo began to pee over the side of the arena in front of the entire audience, Hack approached him, angrily commenting how there are people who were trying to fight seriously, and that he had no choice but to attack the Super Rookie from behind. As the crowd cheered him on, Hack unleashed his attack, only for it to be blocked by Bartolomeo's Bari Bari no Mi ability, which crushed the fishman's hand, severely injuring him. As the spectators and Gatz gape in confusion over what had just happened, Hack writhed in pain on the arena's floor, with the ferocious Bartolomeo leering over him. Continuing what he had started, the vicious Super Rookie then finished Hack off with a brutal kick to the face. Sometime later, he was supposed to receive treatment for his injuries but instead, he was thrown in an underground dungeon filled with broken toys along with some of the disqualified participants. Hack commented that it was safe to assume that they won't be receiving medical treatment at all. After Ricky was revealed to be the former king of Dressrosa, Riku Dold III, Hack was seen secretly speaking with Koala by means of a Den Den Mushi. Hack was later seen in the underground trade port, having been transformed into a toy dog and forced to work. While moving crates, he complained bitterly that Koala was waiting for his report. When Sugar lost consciousness, Hack transformed back into his original form. He was then informed by Koala that Sabo was making his way to the basement.