Sweet Mask/Synopsis

History
Not much is known about Sweet Mask's past. Genos speculates that something terrible might have happened in his past involving Mysterious Beings, due to his merciless nature and hatred towards everything evil. Given his abnormal body, it is possible he was experimented on.

National Superhero Registry Arc
Sweet Mask gets a call from the Hero Association about Genos passing on the test and becoming an S-Class. Later, he visits both Genos and Saitama at a udon shop as they finish an eating competition, asking Genos to speak with privately. He asked Genos how the test was with Genos replying that it was too easy and a waste of time. Sweet Mask stops Genos from attacking him, who was thinking this was a rookie beating, but he only came to congratulate Genos for reaching S-Class and tells him, that he expects great results from him and walks off. After that, he is seen in his penthouse thinking about how interesting Genos is.

Sea Monster Arc
While the Seafolk invade J-City, Sweet Mask makes a guest appearance on a television show to promote his new song. The host of the show asks Sweet Mask what he thinks about the current situation, to which Sweet Mask replies that he is only a hero when he fights evil and that currently, he is here to promote his new song as a singer. However, he goes on to say that heroes have to be tough, strong and beautiful at all times in order to ease the fear and worries of the people. He states that he finds it regrettable that there were heroes who were defeated by evil and stating heroes must improve themselves so that no hero would die in vain in the future. Sweet Mask is called in to advise about Saitama's promotion to B-Class. Sweet Mask replies that they do not need him for such as trivial task as promoting someone to B-Class and even states that he only wants a say in promoting people to A-Class and higher, as they tend to have a strong influence on the association's image. He in the end allows Saitama to be promoted. In the anime Sweet Mask appears in his dressing room along with Bearded Worker to talk about Saitama's promotion to B-Class. Though he only cares about A-Class heroes and above as they are the public face of the Hero Association, feeling disgusted because of the Deep Sea King incident, due to many A-Class and even two S-Class were defeated by the said monster. The Bearded Worker then shows Sweet Mask a video of Saitama defeating the Deep Sea King in a matter of seconds. His discovery on Saitama's true potential sparked minor interest in him.

Alien Conquerors Arc
After the defeat of the Dark Matter Thieves and the fall of their spaceship Sweet Mask appears and mocks the S-Class Heroes for being unable to stop the destruction of A-City much to Metal Bat's anger. Later when they found the remaining minions of Boros while the other heroes wonder what to do with them, he mercilessly kill them saying that "evil" must be eliminated without doubts. The other heroes watch shocked and Sweet Mask lectures Genos about implementing "justice," as well as telling him how disappointed he was with the latter after the Deep Sea King incident and with the most recent destruction of A-City before he walks away. While Genos thought about how, before meeting Saitama, his way of thinking was similar to Sweet Mask's.

The Blizzard Group Arc
Sweet Mask encounters Suppon, a caveman test subject who escaped from his laboratory and had begun to brutally assault civilians. Sweet Mask deemed Suppon as 'evil' and informed him that he was under orders to bring him back alive, but going against these orders, he defeats and brutally kills Suppon in short order, amidst a cheering crowd.

Super Fight Arc
Sweet Mask is seen performing a concert with a band in a concert stadium, but was interrupted by large explosions, signaling the arrival of a Mysterious Being.

Despite the situation, Sweet Mask continues his show, toying with the Mysterious Being before eliminating them with a swift slice and an uppercut. However, the audience believes it was part of the show, and no further questions are asked. After the show, Sweet Mask contemplates about the Monster Association, resolving to take action himself.

Monster Association Arc
Sweet Mask joins Pig God, Child Emperor, Superalloy Darkshine, Zombieman, King, Atomic Samurai, and Puri-Puri-Prisoner in standing against the Monster Association. Sweet Mask effortlessly defeats Goddess Glasses, negating her charm attacks and smashing her against a wall. After, he is faced with Fuhrer Ugly and is overcome by him due to being unable to fight back as a result of his opponent's extreme ugliness, which paralyzes Sweet Mask. Eventually they are separated and Sweet Mask returns to the battlefield to fight the Dragon-Level monsters.

He throws a large boulder at Black Sperm and demonstrates abnormal physical traits over the course of the battle such as being able to withstand almost lethal attacks with ease and reattaching his arm after it was shot off. Veins and scars cover his body as a result of his powers. Sweet Mask aids Child Emperor in his plan to defeat the monsters, fending off legions of the multiplying Black Sperm whilst avoiding attacks from Evil Natural Water, which impresses Pig God and Atomic Samurai.

Later he is confronted by Garou after he destroys Golden Sperm with ease, remarking on Garou's power. Garou mocks Sweet Mask's views of justice and beauty and proceeds to fight the remaining S-Class Heroes. Though Sweet Mask and the others are able to land several blows on Garou, he is able to defeat them all, striking Sweet Mask brutally across the face.

After Saitama defeats Garou, a recovered Sweet Mask yells at him to kill him and says that he cannot be allowed to live after all of his crimes. He then implores Bang to do the same, offering to carry out the execution himself. He is prevented from carrying this out by the child Garou brought with him and is outraged when he is able to escape.

Psychic Sisters Arc
Sweet Mask is in a studio pondering the thought about the beauty of strength and Saitama. He is asked by the Hero Association to look over a group of young and new C-Class Heroes named the Sugar Pop Boys who are in the entertainment industry. However, he declines, finding imitations to be 'ugly' and a waste of time. When the group insults him and challenges him, Sweet Mask is able to intimidate them with a frightening aura, forcing them to cower in fear while adamantly voicing none of them had a true beauty like Saitama, which was overwhelming power. During this time, a strange vein appears on his neck.