Tamiya Gantetsusai, originally named Tetsunosuke, is a prominent character from Hell's Paradise. He is a legendary swordsman of immense renown, known by the epithets the Blade Dragon and Eight Provinces Unparalleled. Standing at an imposing 222 centimeters, his appearance is that of a rugged and seasoned ronin, marked by a full beard and the scars of countless battles. Before the events of the story, his life was defined by the search for worthy opponents, as he had grown bored with defeating ordinary men. He eventually became a retainer for a local daimyo, but his freedom was lost due to an act of impulsive pride. After his lord drunkenly suggested that Gantetsusai could not actually slay a dragon, the swordsman became enraged and used his blade to slice the dragon-shaped gate of the lord's mansion in half. This act of destruction of property, rather than any act of murder, led to his capture and death sentence.

Gantetsusai possesses an audacious, fiery, and unapologetically proud personality. He is a man of grand ambitions, and his primary motivation is not survival itself but leaving a mark on history. He views the concept of physical immortality granted by the Elixir of Life as meaningless. For him, true immortality is achieved when one's name and deeds are recorded and remembered across the ages, becoming a legend. This desire to perform extraordinary feats and be engraved in the annals of history is the driving force behind his actions, far more than the promise of a pardon from the shogun. He finds genuine thrill and enjoyment in combat, often smiling even when critically injured, as the adrenaline makes him feel stronger. While he has a brash and seemingly simple-minded approach, often preferring to rush headlong into battle, he also possesses a remarkable and sharp intuition that allows him to grasp the essence of perilous situations. He has a pragmatic side, willing to use whatever means necessary to survive, yet he also shows a spontaneous benevolence, particularly towards women and children, feeling genuine disgust at the mistreatment of the young girl Mei by the island's masters.

In the narrative, Gantetsusai is one of the death row convicts sent to the mysterious island of Shinsekyo as part of the Vanguard Party. He is assigned to the watch of Yamada Asaemon Fuchi, an executioner who specializes in autopsies and shares almost no personality traits with his charge. Where Gantetsusai is loud and boisterous, Fuchi is morbid and calm, leading to a dynamic of mutual misunderstanding and eventual respect. Upon arriving on the island, Gantetsusai immediately demonstrates his survival instinct. After being bitten on the left hand by a venomous insect, he does not hesitate to sever his own hand to prevent the poison from spreading or causing a fatal transformation. He later receives a steel prosthetic hook from Fuchi, adapting to the handicap with impressive speed. His primary role in the party evolves from a lone combatant seeking strong foes to a key member of an alliance. After a tense initial encounter with the shinobi Gabimaru, Gantetsusai agrees to a temporary truce and alliance to survive the island's dangers. This partnership expands to include Fuchi and Mei, forming a formidable group. He also takes on a mentoring role for the young Tōma, acting as a temporary teacher to help improve the boy's swordsmanship.

Gantetsusai undergoes notable development through the crucible of the island's battles. His greatest weakness is revealed to be his stubborn reliance on overwhelming force. For years, his philosophy was that the best way to win was to strike an enemy repeatedly and with maximum destructive power until they fell. This brute-force mentality, however, proved insufficient against the supernatural beings known as the Tensen, who possess incredible regenerative abilities. A flashback reveals a confrontation with his old master, who defeated the younger Gantetsusai and criticized him for becoming greedy for more strength. The master taught him that one must let go of this greed and focus on a single point, for the more one accumulates, the harder it is to see clearly. In a critical battle against the Tensen named Tao Fa, Gantetsusai finally understood this lesson. Instead of wildly cutting his opponent to pieces, he had to be precise. As his partner Fuchi held the enemy in place, Gantetsusai recalled his master's words and used precise Tao sensing to land a single, flawless strike that cut Tao Fa's tanden, the source of her power, without harming his ally. This marks a shift from a purely physical powerhouse to a more focused and controlled warrior.

Gantetsusai's abilities are terrifyingly effective. He is a master swordsman whose skill is so profound that he can generate compressed air slashes capable of cutting through enormous creatures and the enhanced bodies of the Tensen. His physical strength and endurance are immense, befitting his massive frame. He is proficient with his sword even after losing his left hand, showing no decline in his lethality. In terms of the mystical energy known as Tao, Gantetsusai discovers his affinity is Fire. He learns not only to wield this power in combat but, more importantly, to sense it. This ability to perceive the flow of Tao is what ultimately allows him to execute the precision strike against Tao Fa, proving that his growth on the island transformed him from a reckless brawler into a truly dangerous and disciplined master of the blade.