One-Punch Man's Genos is one of the most renowned cyborgs in the world of anime and manga. The monster-killing cyborg was introduced seeking power in the name of justice. He has eliminated numerous monsters during his tenure as an S-Class hero, while also serving as Saitama's apprentice and aide. Interestingly, the hero has always been shown as a cyborg, and the reason for his cyborgification was explained in two skippable blocks of texts during the early chapters of the manga.
Genos was already strong when he debuted, but he still failed to eliminate Mosquito Girl. Ever since then, he received an ample amount of upgrades with the help of Dr. Kuseno. His newer versions allowed him to fight on equal grounds with nastier beings like the Monster Association executives. The more One-Punch Man's plot progresses, the more cyborg he becomes. And knowing his roots requires going back to the beginning.
Genos first appearance was when he went to Z-City and fought Mosquito Girl. Even though he lost to the monster, he had the privilege of witnessing Saitama in action, which also catalyzed him to seek Saitama's tutelage. The cyborg managed to convince the overpowered baldy by offering to pay rent, though that wasn't his first attempt. He told his soon-to-be master about his past in two temptingly-skippable walls of text in Chapter 7 -- texts that even Saitama couldn't be bothered with.
Genos explained that he used to be a normal human, but a runaway cyborg attacked and razed his home when he was 15. Even though he somehow survived the incident, the entire town was turned into ruins. Dr. Kuseno happened to pass by his town as he tracked the berserk cyborg, and Genos pleaded to the genius inventor to modify him. The doctor obliged. Genos then promised his savior he'd be a cyborg for justice and put an end to the crazy cyborg's rampage in the future as well.
The cyborg continued and told Saitama of his evil-eradicating exploits during the four-year span of being a cyborg, but the overpowered baldy was too impatient to hear the end of it. He told Genos to summarize it in or under 20 words. The gist of it is that Genos has still yet to accomplish exterminating the rampaging cyborg and he required more power to eventually face it in the future.
Genos' just personality roots from a mixture of youthfulness and a heavy sense of responsibility. His tragic past forced him to throw away his humanity and continue living as a cyborg. Fortunately, he was lucky enough to meet a dependable master and trustworthy companions. Since Geno's revenge is yet to unfold, fans can only patiently wait for future updates of One-Punch Man.
