He was portrayed by Mike Vogel.
Zack Shelby was the deputy of the White Pine Bay Police Department, but was also secretly involved in a sex slave operation under the direction of a man named Jake Abernathy. He had even kept a young Asian woman named Jiao locked in his basement where he subjected her to drugs and raped her on a regular basis. One of his partners in crime was an equally violent man named Keith Summers.
When Norma and Norman Bates moved to White Pine Bay, Keith had threatened them, telling them they knew nothing about running the hotel and that it has been in his family for generations. He leaves once Norma threatens to call the police. However, one night when Norman sneaks out of the house to a party, Keith breaks in and rapes Norma but is knocked out by Norman once he arrives home just in the nick of time. They tie his hands together, but Keith recovers and taunts Norma about raping her and how she "liked it". Infuriated for feeling like a victim, she stabbed him multiple times, killing him. Worried she'll go to prison for killing him out of revenge and not self defense, her and Norman drop his body in the bay. However, Shelby and Romero found Summers' hand in the lake. There were carpet fibers caught in the watchband, and Norma and Norman were changing the carpets in the motel when Shelby and Romero had visited. The next day, Shelby and Romero return to the motel with a search warrant. Because Norman kept Keith's belt under his bed for unknown reasons, Shelby found it and hid it from Romero to protect Norma, seeing as to how they have developed a relationship since he has been working on Summers' case.
Although he earns Norma's trust, the same cannot be said with Norman. When Norman breaks into Zack's house to get the belt back, he finds Jiao and manages to escape with the help of his brother Dylan, without Zack knowing about it. However, he could not help Jiao escape as well.
Norman has tried endlessly to convince Norma of Zack's actions but to no avail. However, Norma checks in Zack's basement one night they're sleeping together, but does not find her due to him moving her to a boat. This effects Norman and Norma's relationship and it only gets worse as she forces him to hang out with Zack on a regular basis to get used to him.
Once Norman and his friend, Emma, find Jiao and rescue her to take her to the apartment, they manage to convince Norma the kind of person Zack is once they introduce the two of them to each other. The next night, when Norman and Dylan go to get back Keith's belt, Zack arrives at Norma's house for another night together. However, he hears the water in the pipes surrounding the apartment as Jiao is taking a shower. Once he knocks on her door to her room, Jiao answers and tries to escape into the woods but is chased down by Zack and shot to death. He returns from killing her and takes Norman, Dylan and Norma hostage in their house, badgering them about how he could have taken care of Norma and that the reason things are not working out now is all Norman's fault. He then begins to beat Norma, which enrages Norman, causing him to attack him to protect his mom. Norman is knocked out, but Dylan engages with Zack in a shooting spree that ends with Zack getting shot to death by Dylan.
Even in death, Zack manages to effect Norma and her life, as his boss, Jake, comes to the Bates Motel only to continue the sex slave business. Norma kicks him out of her motel and later he comes back to threaten her by placing Shelby's corpse in her bedroom.