Initially posted on AO3 under the same user name, story content baised off of a post on tumblr (visit my AO3 to see the link for that on this story)
He had always been able to see it, just before it was only in flashes. Spots of red that flashed in the corner of his eye. As a child he had wondered why people were tying red strings to themselves, but after asking his parents once he realized that they could not see what he did so he kept quiet about it, ignored the little red strings that he could occasionally see.
He remembered when he was a kid he had gotten a glimpse of his own parents strings. He had noticed that they did not connect, they trailed off in opposite directions. In a way, he felt that made sense, like his mum and dad were not truly happy together, that they could have been happier, had they only not been together. But Saitama was selfish and he never told his parents these facts, never told them that he could see the flashes of fate, not that he knew it was fate at the time.
In his adult life he had started to see them binding the people around him, the happy couples and the people that seemed to have the best time while at work would have those flashes of red in the corner of his eye. He had tried so very hard to catch a glimpse of red on his own hand, but had never seen it. For some reason his seeming lack of a point to connect him to others saddened him. He became disillusioned with the world, unable to find himself caring when he was fired. Unable to bring himself to care even when faced with a giant crab-man with sharpied on nipples.
He felt as if he could start to care when he saw that big-chinned brat in the park, the same kid that the crab-man had been looking for. He felt a tugging at his heartstrings as he tried to convince the kid that he needed to get somewhere safe. Felt a rush of exhilaration when he was fighting the crab-man. But just as it showed up out of the blue did the feelings, the emotions leave him. He felt emptier than ever, but he had made a resolution to himself, He would train, he would give his all into becoming a hero, in the hopes that he could feel those lost flashes of emotion.
He threw his all into his training, forgetting the red flashes that he could still see. Ignoring the happy people connected by strings only he seemed able to see on occasion. With time he didn't notice any at all, focused as he was on his training. He was not destined to see them again, not until that day anyway.
Saitama awoke as he did every morning, slowly and reluctant to get from his marginally-warmer-than-the-room futon, reaching a hand up to run a hand through his hair, forgetting for a second that it had all fallen out yesterday. He groaned as he rolled over, pushing the heels of his hands at his eyes slightly, trying to rub the sleep out of his eyes. Rolling from his bed he stood, still bleary eyed with sleep, and went about his morning routine, bathroom, wash hands, brush teeth, splash water on face to wake-up. He paused at the last step though, staring at his hands in shock. There tied neatly to his left pinky finger was a bright red string, taunting him.
In anger he attempted to grasp the string, t tear it from his hand, to make it stop taunting him, but his hand passed through. No matter where he tried to grip it, it would not let him hold it. He attempted to cut it next, only to yield the same results. It would not break nor budge, it could not be cut, and he had crushed the scissors in his frustration. So Saitama closed his eyes, in the hopes that the string would go away if he looked away, even though he knew deep down that would not work, after all he had only seen glimpsed from the corner of his eye before. He could tell that just as his strength had increased, that this strange, somewhat fleeting power had increased too, though he did not have the faintest idea of what to do with the knowledge, or even really what the string was.
A new light of determination kindled a spark in his soul. Forgetting the rest of his morning routine Saitama exited the bathroom and grabbed his laptop, pulling up the internet and searching for an answer to the mystery of the red string that would not leave him be.
He found it after three hours of searching. The Red String of Fate, a mythical string that would connect a person to their soul mate. The tangles would note significant people or places. Saitama let out a soft hum and leaned back, looking down at his left hand as an idea formed in his mind.