Berserker Vermelho, whose true name is Spartacus, is the Berserker-class servant of the Red Faction in the Great Holy Grail War. Spartacus was a Thracian gladiator who led a massive slave rebellion against the Roman Republic in 73 B.C. After escaping a gladiator training camp with seventy-eight comrades, he repulsed a Roman force of three thousand men, inspiring a growing army of slaves to join his cause. His rebel army achieved successive victories against Roman forces but was ultimately betrayed by pirates while attempting to cross to Sicily, leading to his final battle where his body was torn apart and never recovered. His first and only loss in life became his legend, carving his name into history as a symbol of hope for the oppressed.

In appearance, Spartacus is a giant of a man standing over two meters tall, composed entirely of pure muscle. His pale skin is covered in innumerable scars that testify to the countless battles and rigorous training he has endured. Leather straps coil tightly about his body and face, yet he maintains a constant, unsettling smile through it all, as if enjoying even the constriction. His scar-riddled face remains composed and smiling whether he is striking an opponent or being struck himself, which many find deeply frightening.

Spartacus is driven by a single, unwavering conviction to revolt against oppressors and protect the oppressed. He never questions his belief that enduring hardship is a strength of humanity and will always choose the path of suffering, following the most virtuous option available to him. He refuses to strike an enemy at a numerical disadvantage and will only fight back against those stronger than himself. After stopping an enemy's attack, he strikes back and turns the tables, grasping victory only at the last possible moment, a reversal strategy that made him extraordinarily popular as a gladiator in life.

Despite being summoned with Mad Enhancement at EX rank, Spartacus can speak fluently in a highly advanced dialect and can articulate complex ideas, such as proclaiming that those who are bound are strong because they accumulate anger and sadness until they are set free. However, true mutual understanding with him is impossible because he is fixated on always making the most difficult choices, which means he never listens to others and cannot be controlled. Rather than actively disobeying commands, he cannot truly understand them in the first place, as his mind can only process the world through the lens of rebellion. He identifies the Black Faction as his sought-after oppressors while recognizing the Red Faction servants as not being enemies, but he will not consider strategy and may turn his anger on any ally who attempts to forcibly stop his march toward battle.

Spartacus is entranced by combat, displaying a wide grin as he both deals and receives blows from start to finish. Rider of Black describes him as someone born to fight and kill, not enhanced by the Berserker class but rather unfit for any other class from the beginning. He fights with an expression of ecstasy that unnerves both opponents and onlookers, even causing those with diluted emotions to become infected by his madness and flee. The constant smile during battle leaves even his fellow servants chilled, and Rider of Black, who faced giants and rampaging beasts in life, admits to being frightened by Spartacus smile. The only thing that changes his expression is the idea of being enslaved, which transforms his perpetual smile into an expression of murderous rage, as he considers enslavement a fate worse than death.

Spartacus has no desire to seek the Holy Grail. His dearest wish is simply to proceed to fields of battle. For him, the battlefield is a place where only the weak and the strong exist, a place full of the everlasting pain and trials he desires. He is described as a fool but not delusional in his madness. With what others consider absolutely perfect logic that has no flaws, he has forgotten that all that awaited his own suffering was a cruel and tragic death. His will to keep moving forward goes beyond hatred or conviction. In spite of his madness, he has carved an unbreakable resolve onto his own heart. Lancer of Black describes Spartacus rebellion as the embodiment of a noble spirit that cannot accept that the strong will always trample the weak. He fights until he has turned the strong into the weak, though he does not fight out of altruism for the sake of the weak but rather for a world where all are equal.

Within the story, Spartacus is originally summoned by a master of the Red Faction, but he frequently rampages in search of an adversary for his rebellion. Caster of Red reveals the location of the Black Faction headquarters to him, and Spartacus begins marching there directly despite the objections of his allies. Even Command Spells would only delay the inevitable, so the Red Faction must follow in his wake to manage the consequences. The Black Faction takes notice of his approach and plans to capture him. He carves a path through their homunculi and golems, even continuing to fight after Rider of Black uses a Noble Phantasm to force his legs into spiritual form, dropping him to the ground. Overwhelmed and pressed by massive golems, he continues to rip through them and proceed toward the castle.

Ultimately, Spartacus is captured by the Black Faction through a combination of Vlad III Noble Phantasm and Avicebron golems melting on top of him. Rather than remaining with the Red Faction, he forms a contract with Caster of Black. Throughout the war, his role is defined less by personal relationships and more by his ideological opposition to anyone he deems an oppressor. His key interactions are primarily combative, and his presence forces both factions to adapt to his uncontrollable nature rather than the other way around. He demonstrates little traditional character development, as his singular conviction is absolute and unchanging from the moment of his summoning until his eventual destruction.

Regarding notable abilities, Spartacus possesses the personal skill The Honor of the Battered, which quarters the magical energy cost required to heal his wounds. The more injured he becomes, the more his combat focus heightens, allowing him to tap into his full potential. In fact, if he is not injured, he cannot fight at full strength. This reflects his fundamental premise that one must first conquer the pain inflicted by the enemy to attain absolute victory.

His Noble Phantasm, Crying Warmonger: The Howl of the Wounded Beast, is a permanently active ability that converts part of the damage inflicted by enemies into magical energy and accumulates it within his body. The efficiency of this conversion increases as his physical strength decreases. The accumulated energy can be used to boost his abilities, and if he is damaged to the point of near death, he may store an enormous amount of prana capable of destroying everything before his eyes. As he continues to absorb energy, his mana saturates the surrounding area, allowing him to damage even servants with debris thrown up by his attacks. However, absorbing too much energy causes his body to begin over-regenerating, growing to a massive size while losing human traits as he sprouts additional limbs and dragon-like heads from his shoulders. Eventually, his body becomes too heavy to support itself and collapses, detonating in a massive explosion equivalent to an Anti-Fortress Noble Phantasm that can cover an entire battlefield and annihilate fortifications.

His EX rank Mad Enhancement trades his sanity and reason for power, ranking up his parameters while depriving him of most of his reason. Although he can speak and converse, he has lost all tactical ability and remains fixated on always making the most difficult choice. He can recognize others and converse, but only within the context that his current course of action is the correct one, making any form of strategic coordination or mutual understanding impossible.