Meet Me Halfway, chapter 2:
The Medic grimaced. “Dragels usually have three parents,” he explained. “A Bearer, a Sire, and a Third. I would’ve asked if you had a mentor, but your name is nowhere in our records and my search included mentor-student assignments. If you had a mentor assigned to you, it would’ve shown up, and one of the main reasons a dragel wouldn’t have had a mentor assigned to them is because they still have dragel-born immediate family members.”
Harry glanced down with a frown. “My mother and father are dead,” he said dully. “Have been ever since I was a baby. I never knew anything about a third possible parent.”
“Do any possibilities come to mind? An adult that was close to you, who would have looked to be about their age most likely, who you felt a connection to or who likely made plans to be in your life as much as possible.”
Harry’s frown deepened, the question bringing up painful memories, even though it didn’t quite fit with what the man was describing. Sirius. Remus. Snape, even, since the man went to such lengths for his mother. “Some. But they’re all dead now too.”