Berserker is the Berserker-class Servant of Illyasviel von Einzbern in the Fifth Holy Grail War. His true identity is Heracles, the most celebrated hero in all of Greek mythology. He is a demigod, born as the son of the king of the gods, Zeus, and a mortal woman named Alcmene. His life was a cycle of immense achievement and profound tragedy, most notably the madness inflicted upon him by the jealous goddess Hera, which drove him to kill his own wife and children. To atone for these sins, he undertook the legendary Twelve Labors, a series of impossible tasks that he completed through a combination of immense strength and cleverness. Upon his death, his heroic deeds were recognized by the gods, and he ascended to Olympus to live among them as a deity.

Berserker is a colossal figure, towering at over two and a half meters tall with a dark complexion and a physique like a bodybuilder’s statue. His wild, black hair rises like a lion's mane, and his eyes are heterochromatic, with one being red and the other gold. Due to the influence of his Mad Enhancement, thick veins are visible on his face, contributing to his monstrous and fearsome appearance. He is typically bare-chested, wearing only a dark battle skirt and armored bracers on his wrists and ankles, and he carries a massive, irregularly shaped weapon known as an axe-sword.

As a result of being summoned in the Berserker class, Heracles is afflicted with Mad Enhancement, which seals away most of his rationality and emotions in exchange for a massive boost to his physical parameters. Throughout most of the story, he acts as a furious beast, utterly silent and incapable of complex speech or thought, acting solely on instinct and his Master's commands. However, glimpses of his original heroic nature remain. Despite his insanity, his combat instincts are so deeply ingrained that he retains incredible martial skill, flawlessly deflecting attacks and fighting with a strategy born from his battle-hardened reflexes. The core of his being, his will, is never lost. This is most clearly demonstrated in his fierce and unwavering desire to protect his Master, Illyasviel, for whom he shows a loyalty that transcends his maddened state.

The Einzbern family, seeking to avoid a repeat of their betrayal by a previous Servant, purposefully summoned Heracles in the Berserker class to ensure he would be a powerful yet obedient "dog". He is summoned two full months before the Fifth Holy Grail War begins, earlier than any other Servant, and he and Illya bond while surviving harsh training in a monster-filled forest. His primary motivation, instilled by his class and his own will, is the absolute defense of Illya, whom he seems to regard with a protective, paternal affection. Berserker acts as an overwhelming force for the antagonists in the early stages of the story. His first major confrontation is a devastating attack on the protagonist Shirou Emiya, Saber, and Rin Tohsaka, where he effortlessly defeats Saber and horrifically wounds Shirou. His presence establishes the immense power gap the heroes must overcome.

Berserker's single most important relationship is with his Master, Illyasviel von Einzbern. Despite her often treating him with disdain and ordering him to fight while she watches, there is a deep, unspoken bond between them. Illya can interpret his grunts and silences as if they were having a normal conversation, and he will fight against impossible odds to prevent her from coming to harm. In many of the story's bad endings, he is the instrument of Illya's cruel and deadly games, but in the main narrative, their partnership is one of the most potent in the war. His other key relationships are adversarial. He sees Saber as a worthy opponent, and in the Fate route, he shows a moment of sanity before his death, questioning her about the sword that defeated him, Caliburn, acknowledging its surprising power. He also has a notable confrontation with Archer, who, through clever tactics, is one of the few Servants capable of taking multiple of his lives in a single battle.

Berserker's role in the story varies by narrative route. In the Fate route, he serves as a powerful mid-boss, ultimately defeated through a combination of Saber's strength and Shirou's projected Caliburn, which takes all his remaining lives at once. In the Unlimited Blade Works route, he battles Archer to a standstill, losing several lives before Illya is captured, effectively removing him from the war. In Heaven's Feel, the darkest route, he suffers the most tragic development. After Illya is taken, Berserker is swallowed by the corrupted Holy Grail's shadow and transformed into a mindless, blackened puppet. In this state, he is forced to fight against the heroes, a perversion of his heroic nature that highlights the route's themes of despair and corruption. Even in this fallen state, the smallest fragment of his original will remains, as he hesitates for a fatal moment before delivering a final blow against Illya.

Berserker's fearsome reputation is backed by several exceptional abilities. His primary Noble Phantasm is God Hand: Twelve Labors. This is a passive, constantly active ability that confers three effects. First, it grants him a conceptual defense that nullifies any attack that is not of the highest rank, specifically A-rank or above, regardless of the attack's pure destructive power. Second, it gives him a stock of eleven extra lives, meaning he must be killed twelve times in total to be truly defeated. Finally, after he recovers from a wound inflicted by an attack, he gains a high degree of resistance or even complete immunity to that specific attack, forcing opponents to use a wide variety of different methods to kill him. His other personal skills complement this defense. He possesses Battle Continuation at a high rank, allowing him to briefly continue fighting even after suffering fatal wounds. His Eye of the Mind (False) is a remarkable instinct that borders on precognition, allowing his maddened body to flawlessly analyze, evade, and counter attacks, even those from invisible weapons. In life, his signature technique was the Noble Phantasm Nine Lives: Shooting the Hundred Heads, a high-speed attack consisting of nine simultaneous strikes used to slay the Lernaean Hydra. However, due to the restrictions of his Berserker class, he is unable to actively use this technique, though it is utilized by other versions of his character in different summoning conditions.