With a premise as simple as One-Punch Man's, interesting characters have been key to the series' success, and one of the most popular is Genos, Saitama's cyborg friend, roommate, and pupil. Genos, like many of the S-Class heroes in One-Punch Man, wields an incredible amount of power as a result of his cybernetic enhancements, but just how did he wind up like this in the first place?

Genos and Saitama initially meet by accident, while the already cyborg Genos is engaged in battle with Mosquito Girl--a being whom Saitama destroys in a single blow. Eager to learn how he did this, Genos pesters him into taking him on as a student, mostly by offering to help with rent. Despite the absurd power he'd already demonstrated, Genos insists that he needs to grow stronger, and provides a rapid delivery of his backstory that Saitama waves away as being too long. Fans who might actually be interested in learning more about Genos may be a bit disappointed at this, but much of that information eventually emerged as the story continued.

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According to Genos, he was 15 years old, living in a small town with his family as an ordinary boy when a deranged robot/cyborg came crashing into town, destroying every last building and tree in his path. Everyone in the town except for Genos seems to have died in the initial attack, and Genos himself would have perished shortly thereafter had he not been discovered by the "Scientist of Justice" Dr. Kuseno. Dr. Kuseno transformed Genos into a cyborg as well in order to save the boy's life, and since then, Genos and Kuseno have had a father/son style relationship, working together to track down the "Mad Cyborg." Whether Genos' recollection of events is accurate, however, is a question often speculated upon by the fandom.

Very little of Genos' original body seems to be left; while he has a human-looking face, even that is synthetic, and the rest of his body seems to be purely robotic. It's suggested that a brain transplant is the typical means of creating a cyborg, and that Genos presumably still has a human brain hidden in there somewhere. Despite this, he's still shown to eat food from time to time, and has obviously been designed to still be capable of participating in human activities. Turning into a cyborg did not change his personality, according to One-Punch Man's creator ONE, although spending time with Saitama has led him to loosen up a bit. By the time of the series, Genos is 19 years old, and had been adventuring for several years in search of the Mad Cyborg before meeting Saitama.

With vengeance having been his primary motivator, Genos is quick to incorporate components from defeated enemies to increase his power, believing that to be his best way to grow stronger. He's very familiar with other robotic and cybernetic heroes like Metal Knight and Drive Knight, both of whom seem to have some hidden knowledge about the Mad Cyborg. As one of One-Punch Man's most popular characters, fans are certainly hoping to see Genos' character arc eventually brought to a close, but with so many questions surrounding his past, it's hard to say just what that end might turn out to be.

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