Donnie
The Demon is first mentioned during Gruff and Subject’s Underground meeting.
Subject: “Oh… the brother?”
Gruff: “The Demon? Didn’t see him. I still get mixed reports, you know but usually that he isn’t there.
When Joan arrives at the Ruckus, Abarth chides her for following “a voice in The Demon’s head.” Joan locates Hank and tells him, “They found your brother.”
Hank deduces that his brother is not alone, and Joan confirms he is with a desert tribe far to the south. We briefly see the one called the Demon laying semi-conscious among a group of others in a similar state.
When Hank and the Horde arrive at the distant tribe’s camp, the leader states that a brotherhood is a rare and powerful thing, and they respect that bond, but The Demon did not want visitors. Hank insists on seeing his brother, but finds himself intoxicated by a hallucinogen in the drink he was given by the tribe.