Lina knows this man as "one of the clerks in the Supply Depot. He was a quiet, long-faced man who always worried" (4.61). We'd worry, too, if we lived in Ember and had any inkling as to how meager the supplies were getting.
But then he decides to go exploring in the Unknown Regions, and he comes back totally traumatized by what he saw. Lina and Clary see him, "running and stumbling, his arms flopping" (4.59) as he reenters civilization. "He tripped over a hose and crumpled to the ground as if his bones had dissolved" (4.59). He's sobbing the whole time, poor guy. And no wonder—it must be totally terrifying to be exploring without any light or any sense where you're going.
Unfortunately, this experience (and possible abuse by guards afterward) makes him totally lose his marbles. When Lina next sees him, he's "shouting and howling" (5.15) on the steps of the Gathering Hall, going on about the monsters in the Unknown Regions and how everyone is doomed. The poor guy has completely broken down and gone mad. But the fact that Lina is able to just walk by indicates that this isn't a totally unusual occurrence in Ember.