Guts, also known in some translations as Gatsu, Gatts, or Gattsu, is the central character of the story. His life is marked by a brutal beginning, as he was born from the corpse of his hanged mother and taken in by a mercenary band led by a man named Gambino and his lover, Shisu. After Shisu's death, Guts grew up under the harsh tutelage of Gambino, an adoptive father who ultimately felt only resentment toward him. This traumatic childhood, which included being forced to kill for the first time at a very young age and suffering a profound betrayal at the hands of his guardian, forged a deeply solitary and mistrustful young man who held little value for his own life.

Guts' early adult life changed when he was defeated and subsequently recruited by Griffith, the charismatic leader of the Band of the Hawk. For three years, Guts served as the band's raid leader, fighting in countless battles. During this time, he began to form bonds for the first time, developing close friendships with comrades like Judeau and Pippin, a complex, antagonistic relationship that blossomed into a deep romantic love with the female captain Casca, and a powerful, ambiguous friendship with Griffith himself. This period softened his aggressive and lonely demeanor, allowing him to find a sense of belonging and even happiness.

However, driven by a desire to be seen as an equal by Griffith rather than a subordinate, Guts decided to leave the band to pursue his own dream. This departure had catastrophic consequences; a devastated Griffith made a fatal mistake, was captured, and tortured for a year, leading to the Band of the Hawk's downfall. Guts eventually returned to help rescue his former leader. The rescue culminated in the Eclipse, a hellish event where Griffith, pushed to his lowest point, activated a crimson Behelit and sacrificed the entire Band of the Hawk to demonic entities known as Apostles to be reborn as Femto, a member of the God Hand. In the ensuing massacre, Guts lost his left forearm below the elbow and his right eye while trying in vain to save Casca, who was brutally raped by the newly born Femto. The trauma drove Casca insane, and Guts was left with a brand on his neck that marks him for sacrifice, attracting evil spirits to him every night.

After the Eclipse, Guts entered a period defined entirely by vengeance. Becoming known as the Black Swordsman, he donned a black mechanical left arm that houses a cannon and a repeater crossbow, and wielded a massive, impossibly large sword called the Dragonslayer. During this time, he was a cold, ruthless, and deeply traumatized individual, pushing others away and even using innocent people as bait to achieve his goal of hunting and killing Apostles. His only companion was the small elf Puck, who was drawn to Guts despite his harsh treatment. This quest for revenge is constantly challenged by his inner darkness, often manifested as the Beast of Darkness, a feral, dog-like apparition that whispers violent temptations and threatens to consume his sanity completely.

As the story progresses, Guts' motivation slowly shifts from pure, self-destructive revenge to a more protective drive. After being confronted by Godo, a blacksmith who served as a father figure, for abandoning the catatonic Casca, Guts decides to prioritize her safety and well-being over his vendetta. He resolves to take her to the mystical island of Elfhelm, where it is said she can be cured of her insanity. This journey forces him to accept the help of others, forming a new, unlikely family. This group includes the young thief Isidro, the former Holy Iron Chain Knights commander Farnese and her loyal servant Serpico, and the young witch Schierke.

Schierke, in particular, becomes essential in helping Guts manage his most powerful and dangerous asset: the Berserker Armor. This magical suit of armor, given to him by the witch Flora, grants him immense, superhuman strength and the ability to fight beyond the normal limits of his body, ignoring even grievous wounds. However, the armor acts as a catalyst for his inner Beast of Darkness, sending him into a near-uncontrollable, homicidal frenzy that erases his ability to distinguish between friend and foe. When he enters this state, Schierke must use her magic to reach his consciousness and protect his soul from being drowned by his own rage. Guts' abilities, already formidable due to his superhuman strength, endurance, and tactical cunning, are thus amplified by the Berserker Armor, making him a terrifying force on the battlefield, but at a great and continuous cost to his own physical and mental health. The body of a human must struggle constantly against the power of a demon, and Guts' entire existence is a testament to that endless, brutal fight.