Jack Kirstein is a notorious arms dealer dealing illegal arms on the black market, and a terrorist leader. He is the secondary antagonist in the episode Surf N Turf in Lethal Weapon, next to Bennett Hirsch.
He is portrayed by Kwame Patterson.
Kirstein is like any other criminal who is adamant about how getting the crime taken care of. He'll stop at nothing.
Later after finding out a waitress, Natasha Belenko, witnessed a murder at the residence of heavyweight boxing champion Ronald Dawson, where the deal was taking place at first, Hirsch sent Kirstein to go after Natasha at her place. After she was rescued by Murtaugh and Riggs, she was brought to the station where she would wait for Witness Protection.
While Riggs and Murtaugh were continuing working the murder case, Hirsch sent Kirstein to the station to kidnap Natasha and bring her to their hideout.