Who turns out to be the mystery man from chapter 5?

Are you referring to part 1?

Day wakes up in a stranger’s house with Tess by his side. Tess explains that he’s been unconscious for two days, but she’s delivered the plague suppressant to his family. It turns out that young Eden is the one who’s sick. The man hosting Day and Tess lost a son to the plague, and he seems to sympathize with Day’s crimes against the Republic. Day remembers how he took Tess, a starving street urchin, under his wing a few years ago.

As Day and Tess talk, the man comes in and warns them that someone is going door to door offering plague cures. The man believes it’s a trap for Day. Although Day and Tess suspect he’s right, they decide to meet the mysterious person anyway, just in case he really does have the medicine.