"I begged Dr. Kuseno to transform me into a cyborg and was reborn as a cyborg who fights for justice. I made a promise to Dr. Kuseno to find that cyborg one day and destroy him." ~ Genos telling Saitama his story.
Genos, or the Demon Cyborg, is the deuteragonist of the webcomic-turned manga/anime series One Punch Man. He's an S-Class hero of the Hero Association and is the disciple of Saitama, as he seeks to become stronger to kill a rogue cyborg that killed his parents four years ago. He acts as a foil to Saitama in the series.
His character design is overall the best design out of all the One Punch Man characters thanks to his cyborg body.
He along with Tatsumaki, Fang, Mumen Rider, etc are the One Punch Man characters that wasn't mean-spirted to Saitama.
His backstory in the second episode of the One Punch Man show was emotional as it shows that he was originally a human but then got turned into a cyborg after his city was destoryed and he almost died until Saitama punched the Mosquito Girl.
He stands up for Saitama as whenever Saitama is having a hard time fighting, Genos would come along and help Saitama fight the villain like team-work.
Even Genos spawned his own meme too, the one where he got hurt by the rock and Saitama tells him that is he okay.
He and Saitama even spawned an epic fight scene even if the fight was only for training, it's still cool.
He once infamously annoyed Saitama in the second episode by calling him "Sensei" and "master" a lot and infamously bald shamed him in the same episode.
Genos shares the same voice actor with other characters:
Kuki Urie from Tokyo Ghoul:re, Sakuta Azusagawa from Rascal Does Not Dream of Bunny Girl Senpai, Naofumi Iwata from The Rising of The Shield Hero, and Sale from JoJos Bizarre Adventure: Golden Wind for the Japanese audio.
Tanjiro Kamado from Demon Slayer, Koichi Hirose fromJoJo's Bizarre Adventure: Diamond is Unbreakable, Nasa Yuzaki from Tonikaku Kawaii, and Aether from Genshin Impact for the English language.