Berserker is the Servant of Illyasviel von Einzbern in the Fifth Holy Grail War, summoned into the Berserker class. His true identity is Heracles, the greatest hero in Greek mythology, known for his immense strength and twelve legendary labors. Because of the Mad Enhancement skill inherent to his class, he has been stripped of nearly all his sanity and reason, acting in a feral and animalistic manner in combat and communicating only with roars. Despite this lack of rationality, a deep and unwavering loyalty to his master, Illya, remains at the core of his being. His motivation is not complex but absolute: to protect Illya and serve as her invincible shield, following her commands without question or hesitation.

In the story of Unlimited Blade Works, Berserker serves as an overwhelming physical obstacle, especially in the early stages. He first appears to ambush the protagonists, demonstrating his immense power by effortlessly pushing back Saber and shrugging off attacks from Archer and Rin. His most significant role comes later when Illya is attacked at Einzbern Castle by the Servant Archer, Gilgamesh. In this desperate confrontation, Berserker fights not to win, but to protect his master. Knowing he is at a tactical disadvantage against Gilgamesh's long-range Noble Phantasm barrages, he charges forward in a straight line, using his own body as a shield and sacrificing his extra lives one by one just to close the distance. This act showcases not only his incredible endurance but also his defining characteristic as a Servant: absolute, self-sacrificing devotion.

The key relationship in Berserker's existence is with his master, Illyasviel. She is the only person who can control him, and he is her primary means of interacting with the world, even serving as her mode of transportation around the Einzbern forest. Their bond goes deeper than a typical contract, however. It is suggested that when they first met, Illya was afraid of him, but Berserker chose to protect her of his own free will, an act that formed the foundation of their powerful partnership. For Illya, who was created as a tool by her family, Berserker is the only one who offers unconditional protection without any ulterior motive. For Berserker, Illya is the small, precious existence he must guard with his life, and his story arc in this route is the tragic fulfillment of that duty, culminating in his death while still trying to shield her from harm even after all his lives were exhausted.

Berserker possesses several notable abilities that make him a formidable force. His immense strength is his primary weapon, wielded through a massive stone axe-club that can cause massive destruction with even the backwash of its swings. His defining power is the Noble Phantasm God Hand: Twelve Labors. This is a passive, always-active ability that grants him two distinct effects. First, it nullifies all attacks that are not of the highest rank, meaning that any physical or magical attack below A-rank will simply bounce off his skin without dealing damage. Second, it provides him with eleven extra lives, for a total of twelve. Each time he takes a fatal wound, he will automatically resurrect, consuming one of these lives. Furthermore, after he is wounded and heals, God Hand grants him a powerful resistance to that specific type of attack, making it far less effective or even useless against him a second time. The only ways to defeat him are to either kill him twelve separate times with twelve different, powerful methods, or to land a single blow so overwhelmingly powerful that it destroys multiple lives at once. It is this combination of raw power and incredible resilience that earns him the title of the strongest Servant in the Fifth Holy Grail War.