This article contains spoilers from the most recent DCEU film. You have been warned.
Doc is the proprietor of an Asian takeaway shop in Gotham City.
Doc owned a takeaway shop in Gotham City specializing in serving Asian cuisine. He also had a spare room above his shop which he rented out to Harley Quinn after her breakup with Joker. He earned Harley's trust after refusing to sell her out and reveal her location. One day, he was confronted by Huntress, who wanted to locate Harley. He reluctantly gave up her location in exchange for a large sum of money.
His shop was attacked by people hunting for Harley and Cassandra Cain. The two survived and saw Doc packing his belongings into his car. Harley questioned him about it and he revealed to her that he had given up her location for enough money to buy better business premises elsewhere in Gotham. He then left a devastated Harley behind.
Doc is quiet and reserved in his communications. He has an understated approach to business too. He is a seemingly friendly individual who serves Harley her favorite food. She considers him to be one the few people she knows in Gotham City who doesn't have some sort of grudge or grievance against her and thus is understandably shocked when Doc sells her out, though he tells her that it's "just business."