Black Sperm is a Dragon-level Mysterious Being who serves as a major antagonist and later a supporting character. His unusual physiology defines his existence, as his body is composed of an enormous number of individual cells, numbering in the trillions. Each of these cells is a separate, autonomous copy of himself, allowing him to function as a collective entity. This unique constitution grants him the ability to split into countless duplicates at will, a power that makes him nearly impervious to cutting or slashing attacks, as such blows merely cause him to multiply rather than sustain injury. Conversely, he can also merge his cells together to form new, more powerful composite beings.

In his default form, Black Sperm has a diminutive stature with a completely black body and a pale, white face marked by black eyes and a small, tail-like protrusion on his head. His visual design is notably simpler than that of many other characters, which often belies his immense threat level. He possesses a complex personality marked by a volatile mix of arrogance and deep-seated insecurity. He is highly confident in his abilities, so much so that he is willing to challenge far more powerful beings, including the Monster King Orochi. However, this confidence is fragile; he reacts with explosive fury whenever he perceives that someone is looking down on him due to his odd appearance or comical name. Despite this pride, he is a pragmatist at heart and does not hesitate to cooperate with other powerful monsters when facing a common enemy, showing a willingness to set aside his ego for the sake of survival and victory.

His primary motivation throughout the Monster Association arc is to destroy the Hero Association, an objective he shares with his fellow monsters, but he holds little loyalty to the organization's leadership. He does not respect the authority of Gyoro Gyoro, the association's strategist, and he is openly dismissive of the Monster King Orochi, viewing their alliance as a temporary convenience rather than a genuine pledge of fealty. After his crushing defeat, his motivations shift entirely toward self-preservation. Witnessing the overwhelming and incomprehensible strength of Saitama, he abandons any ambition of conquest and decides that the safest course of action is to remain close to this incredibly powerful figure, effectively becoming a pet.

Black Sperm plays a pivotal role in the Monster Association arc as one of its most powerful executives. He is instrumental in the assault on the S-Class heroes, using his replication ability to overwhelm and brutally defeat Atomic Samurai, whose legendary swordsmanship proves useless against an enemy who multiplies when cut. He also clashes with other top heroes, including Tatsumaki, Genos, and Sweet Mask, demonstrating his adaptability and power in combat. When pushed into a corner by the combined might of the heroes, he resorts to his most powerful technique: coalescence. By sacrificing the vast majority of his cells, he creates a single, supremely powerful entity. The first of these is Golden Sperm, a massive, muscular form with a shining golden body that possesses incredible strength and speed. In the manga, he is later able to achieve an even more powerful form, Platinum Sperm, which has a sleeker, more agile build and is created from the fusion of nearly all of his fifty-four trillion cells. In this state, he is fast enough to engage in a high-speed three-way battle with the ninja hero Flashy Flash and a semi-transformed Garou.

His key relationships are defined by his arrogance and shifting allegiances. He holds a contemptuous rivalry with Atomic Samurai, fueled by his fury at being underestimated. He is cautious around the S-Class hero King, whose legendary reputation and intimidating presence give even Black Sperm pause. In contrast, he shows a strange sense of camaraderie with the monster dog Rover, expressing concern for its safety. His most significant relationship develops after his defeat. Following the battle, a single, weakened clone of Black Sperm, now too feeble to even harm a puppy, decides to follow Saitama home. Despite Saitama's reluctance, he installs himself and Rover as the hero's unwanted pets, eventually being placed under the watch of lower-ranked heroes at Saitama's apartment complex.

Black Sperm undergoes a notable development from a terrifying, arrogant monster to a humbled and even comic figure. His initial confidence in his near-invincibility is shattered by his defeat at the hands of Garou, and his subsequent observation of Saitama's effortless victory over Garou forces a complete reevaluation of his place in the world. He chooses survival over pride, patiently biding his time and slowly rebuilding his cell stock by consuming protein, all while living a docile, pet-like existence. In this state, he retains his sharp wit and sarcastic commentary, becoming a source of observation and humor rather than a direct threat.

His notable abilities are all derived from his cellular nature. Replication allows him to increase his numbers dramatically, overwhelming opponents through sheer quantity. This ability also acts as a form of regeneration, as any damage that separates his cells simply creates more independent copies. He can also shapeshift, extending his limbs or growing his body to a larger size to increase his physical power. His most devastating technique is Coalescence, an irreversible fusion of his cells into a single, exponentially more powerful entity. Golden Sperm and Platinum Sperm represent the apex of this ability, granting him physical stats that far surpass his base form and allow him to challenge the strongest heroes. However, this power comes at the cost of his near-immortality, as the fused form can no longer split apart and is therefore vulnerable to being destroyed in a single, decisive blow.