Guts is the central protagonist of the manga Berserk, a character defined by traumatic origins and an unyielding struggle against a hostile fate. His life is marked by continuous hardship, beginning with his birth from the corpse of his hanged mother. He was discovered as an infant by a mercenary's lover named Shisu, who adopted him before dying of plague when he was only three years old. After her death, Guts was raised by her companion, the mercenary Gambino, who became his adoptive father in name only. Gambino treated the boy with cruelty, teaching him to fight from the age of three and eventually selling him to a man named Donovan for a single night, a traumatic event that deeply affected Guts.

Guts' personality is a complex mixture of coldness, suppressed compassion, and barely contained rage. As a result of his abusive childhood and life as a mercenary on countless battlefields, he learned to trust only his sword, believing it was the one thing that would never betray him. For much of his life, he remained distant from others, believing that those who become close to him are destined to share in his misfortune. However, beneath this hardened exterior, he is capable of deep loyalty and care. He has shown moments of vulnerability, including tears and remorse, particularly when his actions lead to the suffering of others, revealing his inherent humanity. His personality undergoes significant shifts throughout his life, from a bright and occasionally humorous young man during his time with the Band of the Hawk to the cynical and ruthless Black Swordsman consumed by revenge, and finally to a more guarded but softened individual as he acquires new traveling companions.

The primary motivation driving Guts changes over the course of the narrative. Initially, after surviving the Eclipse, his actions are ruled by an all-consuming desire for revenge against his former friend Griffith, now the God Hand member Femto, and all manner of demons and Apostles. He adopts the mantle of the Black Swordsman and dedicates two years of his life to hunting down and slaughtering these creatures. However, this singular focus eventually shifts. Protecting the mentally broken Casca, his lover who was traumatized during the Eclipse, becomes his central purpose. He seeks to take her to the mystical island of Elfhelm to restore her sanity, a goal that tempers his vengeful quest and leads him to accept the help of a new group of companions.

In the story, Guts serves as the primary point of view, and his journey embodies the manga's central theme of humanity struggling against an absolute, predestined fate. He is branded with the Mark of Sacrifice, a curse that marks him for death and attracts evil spirits, and is fated to be devoured by demons. Yet he continually defies this destiny, earning the Skull Knight's moniker of the "Struggler" for his relentless will to survive and fight against seemingly impossible odds. His existence turns moments of despair into miracles through sheer determination.

Key relationships are the cornerstone of Guts' character development. His relationship with Griffith is the most significant and complex. Griffith, the charismatic leader of the Band of the Hawk, forces Guts to join the mercenary group after defeating him in a duel. In time, Griffith becomes Guts' closest friend and an object of admiration, as Guts becomes his right-hand man and the captain of the Hawks' raiders. This bond is shattered when Guts leaves the band to find his own dream as an equal to Griffith, a decision that sets off a chain of events leading to Griffith's imprisonment and, ultimately, his sacrifice of the entire Band of the Hawk to become the demon lord Femto. This betrayal transforms their friendship into a relationship of bitter enemies. Another pivotal relationship is with Casca, the Hawks' female captain. What begins as a love-hate rivalry born of mutual respect and jealousy gradually deepens into genuine love and a physical relationship. After the Eclipse, Casca loses her sanity, and Guts' mission to protect and heal her becomes his primary reason for living, representing his struggle to retain his humanity in the face of overwhelming darkness.

Guts undergoes profound development throughout the series. In his younger years with the Hawks, he transforms from a lone wolf into a valued member of a surrogate family. The trauma of the Eclipse hardens him into the Black Swordsman, an anti-hero who is apathetic towards the suffering of bystanders and ruthless in his pursuit of revenge. This path slowly begins to change as he accepts new companions like the elf Puck, the young thief Isidro, the knight Serpico, and the former holy iron knight commander Farnese. Their presence helps to anchor him and prevents him from being completely consumed by his own inner darkness. A major turning point is his acquisition of the Berserker Armor. This magical suit of armor greatly enhances his strength, speed, and ferocity by allowing him to ignore pain and push his body beyond its limits, but at the terrible cost of driving him into a mindless, feral rage and slowly eroding his selfhood. The armor manifests as the Beast of Darkness, a monstrous lupine entity that lives in his psyche and embodies his hatred and bloodlust, constantly trying to overwhelm his consciousness and turn him into a mindless killer.

Guts is notable for his immense physical prowess and distinctive abilities. He possesses superhuman strength, speed, and durability, allowing him to wield the Dragonslayer, an impossibly large and thick slab of iron that functions as a sword, with lethal efficiency. It is a weapon too heavy for any ordinary human to lift, yet Guts swings it with enough force to cleave through plate armor, shields, and the thick hides of powerful Apostles in a single blow. Following the loss of his left forearm during the Eclipse, he utilizes a prosthetic arm that houses a powerful cannon and a repeating crossbow, giving him ranged and heavy-hitting offensive options. He is also a highly skilled and relentless combatant, having been raised on battlefields from the age of six and trained in swordsmanship since he was three. His fighting style is not based on formal technique alone but on a brutal combination of raw power, incredible speed, experience, and an indomitable will to win, often allowing him to overcome enemies far beyond what his human form should be capable of. His most notable ability, however, is not a physical one but his unbreakable spirit to struggle against fate itself, refusing to be defeated by any monster, apostle, or god.