Background and early life
At some point before 2038, Duncan joined the Seraphites and lived on the island a short boat ride from Seattle. He followed the Prophet when she was alive, eventually becoming an elder who regularly preached to the Seraphites living on the island. He gave regular sermons at the island's main city, Haven.
Events of The Last of Us Part II
In April, 2038, Lucia told Marcus to ensure Duncan received the rump from the latest venison ration as a reward for their faithful preaching and leadership towards the Seraphites. Marcus received a note from Lucia ordering his to ensure the venison was distributed to the people they desired it to go to. They also reportedly preached with Elder Constance.
Around this time, Duncan preached in Haven. He emphasized how the Prophet's teachings should still guide the Seraphites in their daily lives and in their battle to cleanse the world of infected and restore balance to the world. He also noted that the promise of a restored world was not guaranteed to occur within his, or the other Seraphites' lifetime, pondering that it may be an eternal struggle.