Readers beware: the protagonist is a legendary beta male the likes of which are only rarely seen in zombie-related media because they usually die first. I'm not sure how this one survived.
@Twi None of the spin-off characters appear in the original. I'm pretty sure it's partly Hanazawa Kengo's attempt to keep the original work's focus alive by worldbuilding. That, and he probably gets some decent moola from milking I Am a Hero.