Bercouli, also known as Bercouli Synthesis One, is a central figure in the Alicization arc, serving as the commander of the Integrity Knights and the oldest living knight in the world of Underworld. His history predates his service to the Axiom Church; he was once a human swordsman and a co-founder of Rulid Village, where he served as its first guard chief. In his original life, he was an unparalleled warrior who loathed the church's rule, but his attempt to cross into the Dark Territory led to his capture by the pontifex, Quinella. She used him as a test subject for experiments on the human soul, or Fluctlight, which damaged most of his memories before he was transformed through the Synthesis Ritual into the very first Integrity Knight.

Physically, Bercouli is an imposing figure, standing nearly two meters tall with a muscular, battle-hardened frame. Unlike his fellow knights, he typically eschews heavy armor in favor of a simple, deep-blue kimono-style outfit. He has short, iron-gray hair tinged with blue and light blue eyes that project an intense, calm pressure, reflecting absolute confidence in his abilities. His body is covered in countless old scars from centuries of frontline combat against the Dark Territory, each mark a testament to his long and violent history.

In terms of personality, Bercouli is defined by immense self-confidence and a pragmatic, battle-hardened pride. He is remarkably composed, often appearing relaxed even in the most intense situations, such as leisurely conversing with an opponent while simultaneously maintaining a complex and deadly attack. Despite his pride as a knight, he harbors a faint but persistent distrust toward the Axiom Church and its elders, though he continued to serve them because he believed peace for the Human Empire was impossible without the church's authority. He is not cruel and shows respect for worthy opponents, as seen when he allows Eugeo to heal his wounds during their duel and expresses relief that his disciple, Fanatio, survived her encounter with the intruders.

As the commander of the Integrity Knights, Bercouli's role is to lead the order in defending the Human Empire from the forces of the Dark Territory. His motivations are rooted in a deep-seated wish for peace and a sense of duty to protect the next generation. While he feels he cannot offer paternal warmth to knights like Alice Synthesis Thirty, he is fiercely protective of those under his command, particularly his direct disciple, Fanatio Synthesis Two, vowing vengeance if she were to be killed. Despite having his memories suppressed, his critical thinking remained intact, allowing him to subtly question the church's directives even as he followed them.

Throughout the story, Bercouli experiences subtle but significant development. His encounter with Eugeo, who wields the Blue Rose Sword from the legend of Bercouli's own human past, triggers fragmented memories, creating a moment of internal conflict. When the pontifex Quinella is finally defeated, he is freed from his petrification and takes full command of the Human Empire's forces in the ensuing war. In the War of Underworld, he faces his ultimate challenge against Dark God Vecta, a super-account controlled by Gabriel Miller. Knowing he cannot win through conventional means, Bercouli sacrifices his own life in a desperate gambit. He endures the loss of an arm and the absorption of his own willpower to land a decisive final strike, fulfilling his role as a guardian who falls to protect those who will come after him.

Bercouli's most notable ability stems from his Divine Object-class weapon, the Time Piercing Sword. This weapon allows him to manipulate time itself. Its Enhance Armament technique allows him to "slash the future," leaving invisible cuts suspended in space that activate when an enemy touches the designated area. His most powerful technique, the Release Recollection known as the "Time Piercing Sword – Back," allows him to "slash the past." This attack targets the exact location an opponent was at up to ten minutes in the past, bypassing all defenses and instantly reducing their life to zero by interfering with the system's records. Bercouli dislikes using this technique, as he feels it negates the effort and skill of both combatants, and he reserves it only for enemies who must be killed at any cost. In addition to his temporal powers, he is a master swordsman with nearly three centuries of combat experience, capable of using Sacred Arts to perform feats like regenerating severed limbs, and he possesses a powerful will that allows him to resist attacks on his very soul through the power of Incarnation.