Adamantine
Genos had welded himself to Saitama's side for a number of reasons. He followed his Sensei, every day, week, month... Lived with him, or near to him, at all times, venturing away only when he could sense Saitama's mounting frustrations, or when Genos was needed to fight. Sometimes he would venture away from Saitama during battles, mostly because he could never grow stronger if Saitama was always there to step in and end the battle with a single punch, but when all was said and done Genos loyally zipped back to his Master's side as a piece of mail with the wrong address on it inevitably returns to its sender.
He'd been asked a number of times why he chose to stay with Saitama.
"You're S-Class, why are you always hanging out with that Caped Baldy character?"
"He's your teacher?! Why him?!"
"You aren't being forced to be with him, are you?!" Genos had been especially annoyed by this question.
Saitama himself had inquired on more than one occasion as to why Genos stuck around. "I really don't understand why you're here..."
When asked Genos responded in a number of ways. He'd insist that Saitama's strength made him a worthwhile teacher, assured others that Saitama was much wiser than he first appeared. Sometimes he'd even respond violently to the more jabbing inquiries, ready to defend his teacher's worth in the eyes of others, prepared to force them to see his Sensei's value if necessary, though Saitama always told him not to fight others over such arguments.
Those were all reasons Genos stayed with Saitama, yes; he was never lying when he said his Master was wise, strong, admirable... But there was another reason. One which Genos had never admitted out-loud to anyone other than Dr. Kuseno; even Saitama himself more than likely didn't know.
It was common-enough knowledge that Genos had lost his family at the young age of fifteen. Prior to gaining a cybernetic body, Genos had been not entirely dissimilar to any other teenage boy. After the incident which robbed him of his family and natural body, he'd become cold, far less emotional than the average teenager should be, calculating, and ruthless, all traits which earned him his hero name of Demon Cyborg. Unlike what some would be lead to believe, it wasn't the lack of a human heart which made him so monotone in emotions, but rather the trauma of the event itself. He was as physically capable of feeling emotions as anyone else: His angry and energetic spouts proved as much. But ever since losing his family he'd become guarded, just as one might expect. Who, after all, could blame someone for closing off their emotions after having lost so much?
That was why when Genos first met Saitama he only regarded the man with respect and awe, but not true fondness. Genos would overreact when Saitama faced enemies, worried that something bad would happen, that Saitama would die. It wasn't until several major battles had gone by before Genos realized that, truly, Saitama miraculously couldn't be defeated! He was unbreakable and, most importantly, death could not take Saitama away from him, to the best of Genos' knowledge. Reliable, constant, sturdy...
That was one of the main reasons Genos stuck with Saitama and refused to have any other teacher. His Sensei couldn't be killed, wouldn't be taken away from him as his family had been. It was a comforting thought for Genos: One that he knew perhaps wasn't healthy, and which was likely unwanted by Saitama himself (to be held in such high regard stressed his Sensei, he knew), yet still, Genos valued the assurance that Saitama wouldn't up and die on him as his family had, so he stuck with him, even when others criticized him for it. Saitama was the one person in the world that Genos didn't have to be afraid of growing close to: Everyone else ran the risk of death. But not his Sensei. Not Saitama.
So, even when they sparred and Genos lost and felt disappointed in himself for loosing yet again, for not being strong enough, a part of him was also always relieved. If his Sensei could still defeat him so easily, then Genos could at least rest assured that if ever an opponent came along which could defeat his Sensei, then Genos himself likely wouldn't live to see another person he cared about die. As if such an opponent could ever even exist...
When shortened to twenty words or less:
Genos stayed with Saitama because Saitama, in return, would stay with Genos. He couldn't be taken away: He was adamantine.
