Yul's creation was many generations ago, in what is now known as "the Technical Age". A period of time when buildings reached into the sky, and machines were used for everyday conveniences. But it all came to a halt when a gate opened, linking 2 worlds, and allowing demons and monsters to pour into this unprepared realm. While resistance was there, most of the population were not trained, or well-equipped enough to fight back, and even those that could, would have to do so in groups to deal with larger monsters. So? The people of that time decided to use these monsters, and any other resources that came at them. The result of their labor were magitech weapons, and the first homunculi. Artificial humans, super soldiers with the powers of monsters. Yul was among that very first batch, and was written off as the weakest, and entrusted with missions of lesser danger, such as information gathering and dealing with relatively feeble threats. Easy, considering at that time, Yul didn't have a will or personality of his own.
Unfortunately, the stronger homunculi, which were deployed to fight at the front lines, fell eventually, forcing the humans of that time to use their most powerful weapons, and hence end the technical age. Yul survived this turn of events, as did many humans, continued his mission, and found out that he could evolve by devouring other monsters. An oversight in his design, which would eventually turn Yul into a walking apocalypse. One however, that was on mankind's side. And although it took many years, Yul eventually fulfilled his main objective. To destroy the gate that linked the worlds.
So... What now? Yul was not prepared to handle a situation like this. Left with no goal, no mission, no objective, Yul finally had some time for himself, and began to think for many years in seclusion. About the world, about himself, about what to do, and all the information he had gathered over the years. He looked human, he felt calm, and... he felt... everything. Emotion, empathy, intellect, a higher understanding of the world around him. Specifically, that if another monster like him was ever allowed to exist, this world would not survive. And as it turned out, that would be quite a shame. When he checked back on humans, they had recovered rather well, build cities, learned to cast magic. Odd as humans as a whole may be, they were entertaining. Fun even. And there were plenty of good people that resounded with his new sense of empathy. So? He made a bit of his strength known on a few chosen occasions, and while not simply living his life, would watch the world to make sure it kept spinning.
"I've killed... More times than I can count.
I've burned empires. Devoured demons. Dethroned gods.
And I did it without any of them even knowing my name. That's what it means to be me. To be the invisible monster who keeps the other monsters at bay."