Ernie was an aide to Joker.
After the Joker supposedly inherited $250,000,000 from King Barlowe, he hired Ernie, an accomplished chauffeur, accountant and human resources manager, organizing the recruitment of a new henchgirl. After Joker found out most of the bills he got from Barlowe were counterfeit, he counted upon Ernie once again. He dressed up as the Joker and partied in the Iceberg Lounge, while the actual Joker planned to rob a money transport. Batman saw straight through the disguise and with the use of a toilet, flushed the information he needed out of Ernie.