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The Walker, an enigmatic figure, a man that is said to look like a run-of-the-mill homeless man that walks around using his powers to manipulate spacetime. With them, he had gained himself the nickname of "The Walker" because he walks through all paths and cannot be stopped by any sort of barrier or wall.
He walks through space and time, of course, so there is nowhere where he cannot walk. He is also said to be an innate nomadic man, he likes to wander around the world aimlessly, although Frank quickly realized that he is not an aimless man at all. With his very words the old man had said that he protected the entirety of Japan from "the outside" as if the entirety of the world was some kind of dangerous place or something, even though there are many other countries.
So what is he protecting? Its certainly unlikely that everywhere except Japan is just some sort of wasteland, right? So what could it even be what he referred to?
Check "The Protagnist's Sister Is Actually the Strongest"!