Jack Fensky was a corrupt police sergeant who attempted to kill Mike Ehrmantraut. Although Fensky's appearance on Better Call Saul is brief, he is a major antagonist, as his actions tear the Ehrmantraut family apart and make Mike the subject of a criminal investigation in the second half of season 1.
While at a bar, Mike notices Fensky and Officer Hoffman across the room. He drunkenly makes his way to their table and throws his arms around their shoulders. Pulling them close, he whispers: “I know it was you.” When it comes time to close the bar for the night, Mike is the last patron to leave. As he stumbles home, Fensky and Hoffman roll by in their cruiser and offer to give Mike a ride. When he declines, they insist. They usher Mike into the back of the police car, and relieve him of his gun. Jovially, Fensky presses Mike about what he said to them earlier in the night. Still heavily intoxicated, Mike declares in no uncertain terms what he meant: He's sure that Hoffman and Fensky were responsible for his son's murder. The cops trade glances as Fensky tells Hoffman to drive to an isolated location.
Hoffman and Fensky take Mike to an abandoned lot far from the city. As Fensky and Hoffman exit the car, stealthy Mike slides a gun out from between the backseat cushions -- he hid it there earlier in the evening when he broke into their cruiser, and stashes it under his jacket now. Fensky and Hoffman pull a wobbly Mike from the car, prop him up against a street lamp, then step away to discuss how to handle the situation. Fensky convinces Hoffman that the safest thing to do is to shoot Mike with the gun they took off him earlier, making it look like his death was the suicide of a grieving father, as no one will ask any questions. Mike, who has only been pretending to be drunk, coolly compliments Fensky on his clever plan. The cops whip around to face him, but it's too late: Mike's already got his gun cocked and pointed right at them. Fensky attempts to fire at Mike, but discovers the gun he took from Mike is empty. Mike shoots Fensky twice in the chest. Hoffman tries to pull out his gun, but Mike quickly dispatches him with a bullet to the head. In the meantime, Fensky is able to draw his duty gun and opens fire on Mike, one of his bullets hitting Mike in the left shoulder. Mike returns fire, a bullet hitting Fensky in the neck. As Fensky tries to crawl away while bleeding to death, Mike slowly walks towards him and steps on his leg, preventing him from reaching for his weapon again. Fensky tries to beg for his life, but an unmerciful Mike finishes him off by shooting him in the head. ("Five-O")