The doctor who was mentioned who seemed older and scarred might be Dr. Sawatari, who performed a brain transplant on the dying Haruki into Kiruko's body (who might have sacrificed herself for her brother). It's consistent with the description of Haruki feeling a heat on his head, and being unable to move or sense, until he's spent a long time recovering and connecting to the new body's nervous system.
We also heard that Robin investigated some disappearances of kids after, and then disappeared himself. He might have been made into a new host for the doctor, which would explain him being a doctor with the Immortalites for a while, though he may have become the doctor's apprentice as well.