The Arrival: Brooks acts as the Sole Survivor's contact while searching for the missing synth. He'll inform them that the synth refugee arrived to Far Harbor as scheduled, but panicked, since he thought he'd been followed, and escaped into the fog.
The Way Life Should Be: If DiMA is tried and executed by the people of Far Harbor and the player character is unable to convince the mob to spare Acadia, Allen Lee will kill Brooks after saying that Arcadia will burn "starting with that synth bastard," referring to Brooks. This indicates that Allen Lee knew Brooks was a synth.
Brooks will accept charge cards and redeems them for 100 caps. Even though Brooks seems to want to keep the information that he is a synth secret, some of the harbormen will sometimes mention that he is in fact a synth.