Don Hagarty is a young homosexual man in Derry and the life partner of Adrian Mellon. He sees Pennywise murdering Adrian under a bridge (after he's been attacked by some homophobic teenagers) and tries to tell the police but nothing comes of it. Don leaves Derry after Adrian's funeral and the trial against his attackers.
Sadie dances with Don at the Sadie Hawkins dance. Don's referenced a couple more times throughout the book as well. (Curiously, throughout 11/22/63, Don's last name is spelled with two G's.)