Born under traumatic circumstances from his hanged mother's corpse, Guts was found by mercenary Gambino's companion Shisu, who rescued and cared for him. After Shisu died of plague when Guts was three, Gambino raised him harshly, training him in swordsmanship from age six. Guts joined the mercenary band at nine, enduring physical abuse and exploitation, including being sold to Donovan, who raped him. Guts later killed Donovan. When a crippled Gambino attempted to kill him while blaming him for misfortunes, Guts acted in self-defense, resulting in Gambino's death. Branded a patricide, Guts fled and was shot off a cliff, surviving to join another band.

Captured after a battle loss during his early mercenary years, Guts encountered the spirit Chitch while imprisoned. She healed his injuries at the cost of her own existence. Guts later escaped and fulfilled a promise to place Chitch's remains in a valley of flowers.

At fifteen, during a siege against the Band of the Falcon garrison, Guts defeated their champion Bazuso. Griffith, the Band's leader, confronted and defeated Guts in a duel, forcibly recruiting him. Initially resistant, Guts gradually integrated into the band, forming connections with members like Judeau, Rickert, and Casca. Griffith expressed ownership over Guts, sharing his ambition to rule a kingdom and showing him his Crimson Beherit pendant.

During three years, Guts rose to captain the Band's raiders. During a Tudor fortress siege, he encountered the immortal apostle Nosferatu Zodd. After wounding Zodd in human form, Guts faced near-certain death when Zodd transformed. Griffith intervened, leading to a joint attack that injured Zodd. Noticing Griffith's Beherit, Zodd ceased fighting, delivering a prophecy warning Guts of inescapable death should Griffith's dream falter.

Guts grew disillusioned fighting solely for Griffith's dream. Following the Band's victory at Doldrey, ending the Hundred-Year War, he departed to seek his own purpose. This deeply affected Griffith, leading to his impulsive liaison with Princess Charlotte. Griffith was arrested for treason and tortured for a year. The Band of the Falcon were branded outlaws.

Guts returned to rescue Griffith from the Tower of Rebirth. He reunited with Casca, sharing an intimate moment before the mission. After freeing the physically broken, mute Griffith, the group faced pursuit. The apostle Wyald attacked, capturing Griffith before being torn apart by Zodd. Zodd informed Griffith the Beherit would return to him.

In profound despair, Griffith activated the Crimson Beherit, triggering the Eclipse. This transported the band to an interdimensional realm where the God Hand offered Griffith rebirth as the demon Femto in exchange for sacrificing his comrades. Griffith accepted, becoming Femto. Apostles slaughtered the branded mercenaries. Guts lost his left arm and right eye attempting to defend Casca, who was raped by Femto, resulting in her mental regression. The Skull Knight rescued Guts and Casca, informing Guts the sacrificial brand would attract nightly demonic attacks.

Equipping himself with the Dragon Slayer sword, a prosthetic arm, and black armor, Guts embarked on a solitary quest for vengeance against the God Hand and apostles, becoming known as the Black Swordsman. This path initially isolated him, marked by bitterness and avoidance of attachments. Over time, companionship with figures like Puck mitigated his isolation, though his drive for retribution remained.