Growing up, Harold was incredibly lonely as he had issues interacting with other people. However, he was very interested in music. He once swatted a fly and realising it was stunned, slipped a thread around it. Harold then believed to have a pet on a string and the closest thing he's ever had to a friend.
After realising the sound of its buzz was a B-Sharp, he craved to have a fly for each note - Eventually gaining a symphony of "friends".
Harold was created by writer Ann Nocenti and penciler John Van Fleet for Batman/Poison Ivy: Cast Shadows