The dude is named Nanimo Tanni (meaning "simply nothing" or "nothing, simply"), and is basically a nobody who works for a construction company that cleans up after monsters and heroes fight. He wants to have a purpose and meaning, and currently feels like he has none. He believes that he will be an unmarked grave eventually, and subconsciously that his value in society is determined by humanity.
So, one day, he's working to clean up the outer shell of Elder Centipede after it escaped Blast almost killing him. He thinks about this and realizes that monsters and heroes will be remembered and have meaning, so he decides to become a monster (since there are less restrictions, and he would be far more memorable if he managed to defeat heroes). So, when Nanimo finally is able to cut part of Elder Centipede's outer shell after a great deal of work and effort, he takes a piece of it and leaves.
Later, he goes to a martial arts tournament, one of the few things that distracted him from his "meaningless" existence. In the tournament is Orochi, as a human martial artist, who proceeded to win the tournament with relative ease. Nanimo decided to make his way down to Orochi to ask him how to grow stronger, since strength would be necessary to being an effective monster.
As Orochi leaves, Nanimo follows him. However, he also sees Psykos alongside Orochi and overheard part of their discussion. As soon as there was mention of her turning him into a monster, Nanimo begged to be made into a monster using the DNA of Elder Centipede he had. Psykos was taken aback by the request, but soon complied.
So, Nanimo got the powers of Elder Centipede, but in spite of his abilities, he was overshadowed by the growth of Orochi, and began to train to grow stronger than Orochi, trying not to lose meaning because of Orochi.
Nanimo would end up becoming a cadre, and would probably die in the Monster Association raid, though I'm still trying to think of who I want him to fight before his untimely demise.