Dosun is a hammerhead shark fishman who serves as one of the officers in the New Fishman Pirates, a prominent antagonist group during the conflict on Fishman Island. At thirty years old and standing over four meters tall, his most distinctive physical feature is his hammer-shaped head, with his eyes positioned at the far edges of the flattened sides. He has light blue skin, long wavy pink hair that reaches his shoulders, a dorsal fin running down his back, and gills on his thick neck. His body is large and broad-chested with very prominent rounded forearms, while his hands and legs appear comparatively small. He wears an unbuttoned dark shirt with T-shaped motifs, light pants and sandals, a sash around his waist, and a large buckled belt that runs across his chest from his left shoulder. On his left arm, he bears the tattoo of his crew's Jolly Roger. His primary weapon is an enormous curved hammer with a long, gnarled handle, which he typically carries on his back or over his right shoulder.

Like his captain Hody Jones and the other officers, Dosun was raised in the Fishman District, where he grew up witnessing instances of human oppression against fishmen. From a young age, he learned to hate humans, and as a teenager, he and his peers cheered the exploits of Fisher Tiger, believing the Sun Pirates aimed to destroy humanity entirely. This indoctrination shaped his entire worldview, though his hatred is described as hollow or empty due to a lack of direct personal experience with humans, yet it remains extremely intense. He is willing to threaten or harm any fishman or mermaid who sympathizes with humans or desires friendship with them. He continues to support Hody Jones even though he knows full well that his captain will not hesitate to abandon or sacrifice his own comrades if it serves his purposes. When his power level runs low, his demeanor shifts into a more melancholic and depressive state.

Dosun has a distinctive verbal habit of ending his sentences with the word dosun, which is an onomatopoeic word in Japanese for a heavy striking or pounding sound. This habit is actually an indicator of his current energy and power level, as his verbal tic changes depending on his physical condition. When at his weakest, he says kotsun, progressing through potsun, posun, tosun, then to his normal dosun. As he gains strength through adrenaline or the consumption of Energy Steroids, his exclamations escalate to dogan, bokan, zugan, bagon, gyaban, and even further levels. This verbal scale reflects his fluctuating combat capabilities, with his power increasing dramatically as his speech patterns become more aggressive.

As an officer of the New Fishman Pirates, Dosun has command authority over lower-ranking members of the crew. Being a fishman, he possesses natural physical strength at least ten times that of an average human, and as a shark-type fishman of his size and rank, his base strength is considerably higher than that baseline. His main combat method revolves around his massive hammer, which he wields with tremendous force. He has demonstrated the ability to smash an entire house off its foundation with a single swing when empowered by Energy Steroids. His known techniques include the T-Shot, a horizontal hammer blow of considerable force, and the Papara Hammer, where he spins his body to build momentum for a devastating strike.

Dosun heavily relies on Energy Steroids, a drug that temporarily doubles the user's strength with each pill consumed but shortens their lifespan as a consequence. He first uses these pills to break free from steel chains after being defeated and bound by Prince Fukaboshi, then consumes more during the main battle at Gyoncorde Plaza to help overwhelm the Ryugu Kingdom princes. However, the side effects of the drugs eventually catch up with him. After his defeat and imprisonment at the end of the Fishman Island conflict, his body rapidly ages and weakens, leaving him physically decrepit, with gray hair, sagging skin, and his former strength completely stripped away.

In the story, Dosun is assigned to seize control of the Water Wheel Village in the northern part of Fishman Island as part of the New Fishman Pirates coup. There, he forces the citizens to step on a fumi-e, a portrait of the late Queen Otohime, as an act of humiliation and defiance. This act enrages Prince Fukaboshi, who arrives and strikes Dosun down, easily defeating him and leaving him bound in chains. The citizens consider using Dosun as a hostage, but he informs them that Hody Jones would not care about his fate. One of his subordinates then feeds him an Energy Steroid, allowing him to break free, destroy a house in a fit of rage, and swear vengeance against Fukaboshi.

He regroups with the rest of his crew at Gyoncorde Plaza, where he and the other officers take more Energy Steroids to defeat the three princes of the Ryugu Kingdom. After the princes are captured and tied up alongside King Neptune, Hody Jones reveals that he was the true murderer of Queen Otohime, a fact Dosun and the other officers already knew and celebrated. When Monkey D. Luffy appears to rescue the royal family, Dosun attempts to stop him but is intercepted and quickly brought down by Roronoa Zoro. He later tries to attack Usopp, believing him to be defenseless, but Tony Tony Chopper intervenes. In their confrontation, Chopper blocks Dosuns hammer strike with a bare-handed punch, shocking the fishman. Ultimately, Chopper transforms into his Monster Point and defeats Dosun with a single blow. After Hody Jones is defeated, Dosun attacks his own fleeing subordinates, ordering them to stay and die so their grudge against humanity can be passed to the next generation. He is subsequently imprisoned in the Ryugu Palace prison. When he regains consciousness some time later, his body has become old and weak from the cumulative effects of the Energy Steroids, though he still expresses a desire for revenge against the Straw Hat Pirates.