Like Adrian Monk, Harold suffers from Obsessive Compulsive Disorder. Despite (or perhaps because) of their commonality, a strong rivalry developed between the two men, mostly over which of them is liked better by Dr. Kroger, or which of them has made better progress in resolving their issues. When they first met each other, they initially got along, but things quickly went downhill when the two argued on how to arrange the magazines in the office waiting room. Monk wanted to organize the magazines symmetrically, while Harold insisted on organizing them by title. (Mr. Monk and the Girl Who Cried Wolf)
In addition to his OCD, Harold also suffers from acute paranoia and narcissism. As a result, he deludes himself that everyone is out to get him, either because of his importance in the scheme of things, or because he inspires outrageous jealousy in everyone else. He often takes great pleasure in "getting" other people before they get him, especially Monk. (Mr. Monk Fights City Hall)
In 2006, much to the dismay of Adrian Monk, Krenshaw took a bullet to save Dr. Kroger's life, and rubbed it in on Monk. (Mr. Monk Gets a New Shrink)
Harold is married and has a son, Jimmy. In 2007, he eclipsed a jealous Monk in fame when the public at large believes him to be a notorious daredevil known only as "The Frisco Fly." In reality, Harold was dropped into the role by his cousin, who was trying to kill him over an inheritance. Unaware of the murder plot, Harold went along with the charade because he loved the public attention, getting a golden opportunity to infuriate Monk, and gaining new respect and admiration from his son. However, he retained his nasty streak: Shortly before his cousin made a second attempt at murdering him, Harold alongside his cousin, after some kindergarden students made a Frisco Fly picture in his honor, cruelly mocked the errors on the picture, causing the teacher to express disgust that he was a member of the schoolboard. (Mr. Monk and the Daredevil)
Following the death of Dr. Kroger, Harold tried hypnotic regression therapy with a new doctor which seemed to cure him of his OCD, inspiring Monk to try it as well. However, the therapy later backfired when Harold's feelings of euphoria led him to strip naked in public, and he was arrested for indecent exposure. (Mr. Monk Gets Hypnotized)
Harold was interviewed for a television special commemorating Monk's achievements as a detective, which Harold derided. In addition, he claimed that he also found a new doctor, not elaborating any further other than that he was "better" than Monk's physician. Mr. Monk's 100th Case)
In 2009, Harold had become a member of the San Francisco city council, where he gleefully voted against Monk's motion to preserve the parking garage where Trudy Monk was murdered, instead of demolishing it to build a children's playground. At first, Monk swayed the rest of the council to vote in his favor, but inadvertently tipped the vote when he insulted one of the initially favorable council members into changing her position. Harold also (thanks to an inadvertent slip from Natalie) learned the name of Monk's new therapist, Dr. Bell, whom Adrian had been bragging about, and announced that he would also be seeing Dr. Bell in the future. However, the council did decide to name the playground after Trudy in her memory. (Mr. Monk Fights City Hall)
Later in 2009, an awkward moment came when "Mr. Monk is Someone Else," when Monk went to Los Angeles masquerading as a mob hit man. Harold, who happened to be visiting Los Angeles, passed out the restaurant where Monk was lunching with several gangsters. Monk managed to stay in character and told Harold he had the wrong man, eventually threatening him with immediate death if he didn't go away. (Mr. Monk is Someone Else)
The two men finally reconciled their differences when Monk's HMO forced him to attend group therapy sessions with Dr. Bell, which included Harold. The two butted heads constantly, even to the point that, when two of their other group members are killed under mysterious circumstances, Harold accused Monk of being the killer.
Both men were kidnapped by the real killer and thrown into a car trunk together, where they both broke down with claustrophobia. Harold admitted that he has greatly exaggerated his own progress overcoming his phobias to goad Adrian, while Adrian admitted that he envied Harold's relative success in going on with his life despite his many fears. The two men realized that they shared many of the same problems, and even overcame their claustrophobia together, before joining forces to escape the trunk and rescue Dr. Bell from the killer.
Afterwards, in an extraordinary gesture, Harold voluntarily transferred to another psychiatrist, to let Monk's "group sessions" with Dr. Bell be individual sessions after all. Dr. Bell said that Adrian had a true friend in Harold, and Adrian did not contradict him. (Mr. Monk Goes to Group Therapy)