Black Sperm/Synopsis

History
Nothing is known about Black Sperm's past or how he came to be.

Monster Association Arc
Black Sperm is first seen present at Gyoro Gyoro's speech at the Monster Association Headquarters. He is one of the monsters who speaks up about serving under Orochi, stating that they are only allies at the moment to crush the Hero Association. He backs off when Gyoro Gyoro reminds the monsters that anyone who can challenge and defeat Orochi can take his place.

Black Sperm also watches as Gyoro Gyoro offers Garou a position as an executive member of the Monster Association. He shows surprise at this and sarcastically quips at Gyoro Gyoro for giving Garou such a lucrative offer.​​ Later, after Orochi and Garou's battle, Gyoro Gyoro calls all the Monster Association executives into the chamber, including Black Sperm. He tells them that he's "figured it out", telling them which S-Class hero each executive should challenge.

Black Sperm is next seen chasing Tareo through the tunnels of the Monster Association base, toying with him and telling him he's going to kill him. He is interrupted by Atomic Samurai, whose slash he manages to avoid. Atomic Samurai is surprised by his speed, but notes that he's just a monster. Black Sperm is enraged, sensing that Atomic Samurai is looking down on him, and attacks him. Atomic Samurai slashes through him hundreds of times, but each time he only splits into more Black Sperm cells. There are eventually so many Black Sperms that Atomic Samurai is hopelessly surrounded, and Black Sperm tells him that this place will be his deathbed. Black Sperm proceeds to completely brutalize Atomic Samurai, outnumbering him and surprising him with his physical strength. Black Sperm is also later seen feeling the impact from Saitama's punch on Overgrown Rover.

Black Sperm's beatdown of Atomic Samurai is interrupted by Tatsumaki, who creates a barrier around the hero and begins pulling him upwards as she prepares to make her move. Black Sperm is confused and frustrated by this, and grumbles as Atomic Samurai manages to get one last slash in before floating through the falling debris.

While the S-Class heroes finish Psykos, Black Sperm and the other Monster Association executives emerge from the rubble. He grabs Sweet Mask before he can attack Homeless Emperor, allowing Fuhrer Ugly to mercilessly dispose of the hero. After watching Homeless Emperor supposedly incinerate Zombieman and Child Emperor, Black Sperm is disturbed by his ally's destructive capabilities. The trio of executives then make their way towards the remaining S-Class heroes, agreeing that whoever kills the most heroes gets to be the new Monster King.

Black Sperm continues to casually watch as Homeless Emperor chases Tatsumaki with energy balls, plotting how to overpower him when the time comes. When he notices Genos and Bang trying to bring the injured Tatsumaki and Tanktop Master to safety, he forms a line of defense to repel them. He then reunites with Atomic Samurai, who uses the sides and hilt of his blade to beat Black Sperm's cells rather than risk cutting them too much. Many of his cells are also immobilized by Tatsumaki, allowing Genos to bring Tanktop Master to safety. Once Superalloy Darkshine begins taunting him, Black Sperm's hot temper begins to surface.

Black Sperm sends a deluge of cells after the heroes, attempting to overwhelm them with numbers, but is repelled at every turn. He considers splitting down to every single individual cell to perform a city-wiping "Sperm Tsunami", deciding against it in favor of merging to become stronger later. Frustrated, Black Sperm harasses Homeless Emperor for sitting on the sidelines and is annoyed when he refuses to join a conflict he has no stakes in yet. The arrival of the Council of Swordmasters continues to push Black Sperm into a tight situation. Eventually, he manages to grab the weakened Tatsumaki, but Genos rescues her and takes a hit for it. Black Sperm then notices the rancid stench of Vomited Fuhrer Ugly's approach.

After Fuhrer Ugly showcases his new powers, Black Sperm is disgusted by his ally's transformation. In retaliation, Fuhrer Ugly begins to consume Black Sperm's cells and challenges him to take him on if he has a problem with it. Reaching his breaking point, Black Sperm decides to merge a large amount of his cells to create Golden Sperm. After birth, Golden Sperm easily pummels Fuhrer Ugly, unaffected by his acidic body due to him having the anti-corrosive properties of gold, with the fat monster then leaving out of mercy as he realizes that he is being outmatched.

When a separate force of ten thousand Black Sperm count consisting of four clones watches Fuhrer Ugly, who is trying to find Bang to claim his revenge, from a distance and plans to take him down, Metal Bat suddenly appears and asks the clones if they know Garou's location. In response, Black Sperm attacks Metal Bat, stating that it doesn't matter to a dead man. The hero quickly retaliates against them with a single swing that annihilates all of the four clones. With Homeless Emperor and Golden Sperm handling the other heroes, Black Sperm fights Tatsumaki and Genos, who are desperately trying to fend off his assault, with the cyborg having all of his limbs torn off. After Genos, despite losing all of his limbs, propels himself and manages to only destroy three clones to get close to Tatsumaki, Black Sperm reveals that with that action, there is only 449,100,712,554 remaining of his cells. As many clones of Black Sperm surround and stomp on Genos, who is using his own damaged body to shield Tatsumaki from the monster's onslaught, King appears with his King Engine rumbling, leading Black Sperm to stop in his tracks. With Evil Natural Water then showing up, Black Sperm, Golden Sperm, Homeless Emperor and the water monster ready to confront the famous hero. After Black Sperm begins to grow paranoid and fearful of King's words and next movements, when he believes that King is really looking down on them, Black Sperm becomes enraged and decides to merge all of the Black Sperm clones together in order to defeat King. All of the Black Sperm clones charge toward Golden Sperm and begin merging with him, while a unit of 100 Black Sperm cells is detached as insurance. As the 100 unit of Black Sperms is confronted by Metal Bat, Black Sperm is in tears. While dodging Metal Bat's attacks, he feels incredible sadness because 53,999,999,999,900 of "him" are being erased as they all fight for dominance, yet he is incredibly happy at the same time because all of them are combining to form one new being, the Platinum Sperm. As the ultimate fusion is about to make his debut, the final Black Sperm clone watches that moment while Metal Bat is about to smash him. Later, Black Sperm is seen splattered on the ground behind Metal Bat.

Psychic Sisters Arc
After the Monster Association incident, it is revealed that a single Black Sperm cell survived. He emerges as Pig God is looking for the last of Evil Natural Water so he can finish it off, but stays hidden. Once Pig God departs, Black Sperm begins to make plans to regain his cell stock, but sees Saitama searching through the rubble for the remains of his apartment. Black Sperm is surprised to notice that the weakened Overgrown Rover is docile with Saitama, and realizes it might actually be safe to stay with such a powerful individual. Black Sperm then follows Saitama home, much to his chagrin.

Once they reach Saitama's new apartment, the security robots made by Metal Knight detect Black Sperm and Rover and attack. Saitama quickly deals with the robots, not realizing his followers are both technically monsters, and Black Sperm sneaks in with him by clinging to Rover's side, now realizing that he's entered the Hero Association's private apartment complex.

Saitama does not want to have to take care of Black Sperm and Rover, so he makes a bet with fellow hero Forte to make him keep them, which Saitama wins. The two formerly dangerous monsters are then entrusted to Butterfly DX and Chain'n'toad, living in a dog house on their apartment balcony.

Supreme Hero Arc
While Butterfly DX, Chain'n'toad, and Forte discuss the Neo Heroes, Black Sperm begins to wonder if the Monster Association really did succeed in destroying the Hero Association after all, smirking to himself. He is seen later commenting on the appearance of 5 Dragon-level monsters.