"- well, Amy Benson and Dennis Bishop were never quite right afterwards..."
--Mrs. Cole (HBP13)
Dennis Bishop was a child from the Tom Riddle’s orphanage whom Tom Riddle once terrorized in a seaside cave (HBP13).
When the orphanage takes a trip to the sea shore, Dennis Bishop and Amy Benson follow Tom Riddle into the cave where he will one day hide a Horcrux. In the cave, Dennis and Amy have an experience that causes them to display noticeable and lasting behavioral changes. They will not reveal what happened in the cave. Mrs. Cole believes Tom Riddle did something to the other two children, but he maintains they had merely explored (HBP13).
Dennis Bishop is an orphan (HBP13).