In Arclight, his role was pretty much defined by and confined to his relationship to his little sister. He stayed behind to protect her and her friends during the first siege of the Arclight, even though it put him at risk to do so.
He was the means through which Marina first saw how seriously the adults in the compound took exposure to the Fade. He was brought into the hospital with severe burns, and she was horrified to hear that the burns had been intentionally inflicted to "protect" him from the Fade. Later in the story, Trey was the one that allowed people to see that there might be more to the Fade than they realized when one healed his wounds.
His role in Meridian stems from those two points of contact with the Fade, and is larger than the one he had in Arclight. The repercussions of his exposure to the Fade serve as the catalyst for Meridian's main storyline. He brings up new questions about the nature of the creatures that destroyed the world, as well as leading the characters down paths that may answer their greatest question -- are they the last people left alive.