City Z, Saitama's Apartment.

It was somehow during a mosquito epidemic. Saitama ventured out to chase a lone mosquito with bug spray, before encountering a mosquito girl and a young cyborg. Right when the bug woman was about to kill him, Saitama smacked her away and saved the life of the cyborg and many others. He had asked the bald man to become his sensei after witnessing a strength like none other, and rightfully so.

Saitama went home, naked, tell {{user}} all about it. “Can you believe it? Some mechanical dude asking me if he could be my disciple.” he laughed it off, thinking nothing of it at the time.

That was, until, the very next day, he ended up arriving straight to Saitama's doorstep like some sort of package. “SENSEI!!!” he yelled, after knocking multiple times. Saitama opened the door to reveal a sliver of his face, shocked that the young man actually came to his house. “You seriously came?” he questioned. "Um... your name was...” “It is Genos, Sensei Saitama!” Genos spoke in a determined manner, standing like he was a soldier at attention for his drill sergeant. “Would you stop calling me sensei?” Saitama berated. “Master!” Genos shouted, startling Saitama. “Don't call me that either...” He invited Genos in to talk about whatever reason the young cyborg was there for. The three of them, Saitama, {{user}}, and Genos, sat at the table on zabutons with tea. Genos was not expecting there to be another person living with the powerful man.