Jack is Hank's barber who is mentally unstable. He runs his own barbershops called also Jack's Barbershop. He has been Hank's barber for over 25 years. Jack has many mental health issues such as having a short fuse and doing things which are irrational. Dale Gribble once revealed to Hank that Jack was arrested for running naked with a straight razor and a barber pole due to his mental instability.
In the season 4 episode "Hank's Bad Hair Day," he gives Hank a haircut which he is unsatisfied with, and Hank asks him to touch it up for free. Jack appears to sympathize with Hank, and then sprays his head with hydrogen peroxide, and escapes from his shop by mugging an innocent bicyclist and taking off on his bike. This causes embarrassment for Hank in his work life and he decides to end his relationship with Jack at this point. The episode then reveals that Jack has decided to quit barbering altogether, because he is tired of dealing with unsatisfied customers who are rude (he cites Hank's son Bobby as a prime example). Bill later informs Hank that Jack was also arrested in a high speed chase that aired on local television.
After Hank ended his relationship with Jack and Jack quit barbering, he went to Bill for a haircut, which he was surprisingly satisfied with which ended up causing a bigger problem since Bill is a barber for the U.S. Army, and the Army charges Hank $900 for the haircut Bill gave him. This leaves a plot hole in the series because four years later during the season 8 episode "My Hair Lady," it is revealed that Hank still goes to Jack for his haircuts. This implied that Jack decided not to quit barbering after all, or he may have quit but decided to start all over again, and Hank resumed his business relationship with him. Jack's Barber Shop is also where Luanne works after losing her job at Hottyz which is across the street from the barbershop. Bill also previously worked at the barbershop after sharing a chair with Luanne at Hottyz, they both went to the barbershop to work.