Bingo! A few days ago I said that "One" reminded me of the killer from the "Lazarus" series (Soryu / "Phantom Killer" / Hundun). The construction of One is pretty much like that, and well, it seems to be the construction of Ijin's antagonist.
While Ijin grew up in the "Camp" thanks to teamwork (and even the "mercy" of Mad Dog), "One," on the contrary, wanted to be "the best of all," and in fact he was, for one simple reason: he killed without mercy.
I went back to rewatch the initial chapters of the series, which reminded me a bit of Ijin's past in the camp. From what I understood, Ijin's time there was the best for everyone because he reacted without hesitation and somehow also knew how to manage his group. However, due to remembering the past of his fall, he decided to leave the camp because he didn't know what to do and didn't want to kill uncontrollably anymore (even though he was more controllable than One).