David Chu is a detective with the Los Angeles Police Department. He was born in the U.S. to parents who immigrated from China. His background has some similarity to Bosch's; Chu was orphaned when he was young. Unlike Bosch, he is a vegetarian.
On 8 September, 2009, Harry Bosch requested assistance in interviewing Chinese-speaking witnesses in the murder of John Li. At that time Chu was assigned to the Asian Gang Unit, and he provided the translations and also explained elements of Chinese culture to Bosch. Together they focused on Bo-Jing Chang as the prime suspect. Hoping to advance the investigation, Chu worked with Monterey Park Police Department to locate Chang. However, Bosch felt that this step was not handled properly, and this led to friction between the two. Bosch also suspected Chu of leaking information to the Triad. It turned out that there was no leak.
Despite his difficulties with Bosch, Chu played a valuable role in the case. His contacts in the Asian community led to the identification of Chang. Then he and Bosch arrested Chang without incident at LAX. When Bosch was in Hong Kong, Chu provided information about a phone number related to Northstar Seafood and Shipping. This led directly to the rescue of Bosch's daughter.
At some time after this, Chu transferred to Open-Unsolved and was partnered with Bosch. Their supervisor was Lt. Gail Duvall. O-U seemed to be a good fit for Chu because he has a strong aversion to seeing crime scenes and dead bodies. He hates going to autopsies. With O-U there aren't any bodies. Chu continued to have some friction with Bosch, largely due to Bosch's lack of communication, but overall they worked well together. Chu's computer skills are especially important because Bosch's are so poor. Chu is described as "lightning quick" on the computer. During this time Chu started dating LA Times reporter Emily Gomez-Gonzmart, but this was kept secret.
On 3 October 2011 Chu and Bosch were assigned a cold hit case in the 1989 murder of Lily Price. Within an hour they were told to go to the Chateau Marmont to investigate the apparent suicide of George Irving. Chu began feeding case details to Gomez-Gonzmart. When she called Bosch to confirm this information, he knew Chu was the source and said he did not want Chu as a partner after these two cases. Chu was extremely apologetic. He broke up with Gomez-Gonzmart and promised Bosch that he would make it up to him. When the detectives were called to a showdown meeting with the Chief and Councilmember Irving, Chu very effectively defended his partner and their work on the case. Chu identified the man known as "Johnny" or "Chill" as Chilton Hardy. They located Hardy's home in Los Alamitos and arrested him. They also prevented Clayton Pell from killing Hardy as revenge. Afterward Bosch said he wanted to keep Chu as his partner.
In 2012 Chu and Bosch were still partners. They had a new Lieutenant - Cliff O'Toole. Larry Gandle was promoted to Captain of RHD. Chu and Bosch were each given Open-Unsolved cases to work. Chu assisted Harry in working the Anneke Jesperson case. Chu ran the possible serial numbers on a Beretta Model 92 but there was no record of the gun in California. Later he researched several men who had been in the 237th Transportation Company in Operation Desert Storm, giving Bosch valuable clues on J. J. Drummond, Carl Cosgrove, and others. At this time Bosch stopped calling his partner "Chu" and started using his first name. It was a sign of greater respect for his partner.