Berserker is Robert the Devil, a medieval knight from the 13th century. His mother, the Duchess of Normandy, sought a child, but found that no matter how much she prayed, she remained infertile. So, rather than make entreaties of God, she chose to ask the Devil for a child. A nameless True Demon answered her request, and she becomes pregnant. The child born was possessed by Demonic instincts, and delighted in terrorizing those around them. For much of their early life, they would commit numerous acts of atrocity, accounts of which would be dismissed as brazen exaggeration, if their occurrence had not been all too real. Yet, bizarrely, the Demonic existence that was [Robert the Devil] did not ascend to the position of a True Demon. One day, after slaughtering the nuns of a convent, Robert the Devil decided to visit his mother. Upon entering their childhood home, they saw the servants flee before them, and the Demon found that they were dismayed at the sight. There, he had an epiphany. The Demonic Existence realized that they were, impossibly, repulsed by their own wickedness and cruelty, and with that revelation, came the realization that they did not wish to be a Demonic existence. For the first time, he feels remorse for his actions. He questions, then, why was he possessed of these urges? Why was he born? He rushed to his mother, intent on discerning the truth of his birth, and he pleaded with his mother for answers. Well, 'pleaded' is a rather strong word. His revelation of humanity had only occurred mere minutes ago, and in his inexperience at being [Human], he decided the best way to convince his mother to tell him the truth, was to draw his sword and threaten his mother with death if she did not tell him. His mother, not wanting to become a shish-kebab, tells him of the circumstances of his birth, how she had called down Satan[It was really just some True Demon, but she didn't know that] to grant her a son. And thus, Robert the Devil resolved to find a way to defeat his own nature.
He reasoned that to defeat the influence of Satan, one would require the aid of God. Since the Lord didn't really have a help line, Robert sought the aid of the Lord's intermediaries on earth, the Church. He came before the Pope himself with his entreaties, and managed to confess each and every single one of his many, many sins. Under the Pope's direction, he undertook years of penance, and among these penances the most notable was his service under the Burial Agency of the Church. A half-Demon fit right in with the Burial Agency's collection of overpowered people, and he fought for the Agency in innumerable missions, purging Demons and Dead Apostles. He forsook all glory, honor, and material gain during his penance, acting as a sort of warrior-monk. Eventually, he retires due to his old age, and lives out the rest of his days as a hermit in a secluded forest.