Mercer Hayes was a Hearst College student and DJ for the campus radio station, KRFF, where he had a live call-in request show named "Club Flush". He also ran an illegal casino out of his dorm room called the Benetian, at 332 Bennes Hall. He appears to have known Logan Echolls before Logan started college, as they talk about a trip they took to Mexico over the summer.
When Logan and Veronica attend his Halloween party, his costume is that of Alex DeLarge from A Clockwork Orange. His casino is robbed by two men in presidential masks, and they insist on taking the cashbox. When he doesn't comply, they abuse one of his dealers. It's only with the threat of violence that he opens the box and hands it over. When questioned about the event by the campus police, he continues to be sarcastic and uncooperative. (President Evil)
Parker recognizes the cologne that Mercer is wearing as being the cologne that her rapist wore. Veronica takes the information to Sheriff Lamb who notes that his officers found empty GHB vials in Mercer's stolen cashbox and he arrests Mercer. Logan insists to Veronica that Mercer was innocent because they were both in Tijuana the night of one of the rapes. Veronica is able to find evidence that Mercer was not the rapist as the radio station database shows that his live request show was on the air the nights of the rapes of Stacy and Parker. (Hi, Infidelity)
Mercer attends a Pi Sigma Sigma party with Logan. He has a brief discussion with Veronica, making a hasty exit when it's apparent she only wants to speak with Logan. Even when Veronica assures him he can stay, he insists on leaving since his radio show is about to air. After the show begins, Wallace discovers a cup that has been drugged that belongs to 'Kim Kaiser', and he leaves with Logan to intercept the rapist.
Unfortunately, Veronica is informed that Kim Kaiser had lent her ID to her little sister, Carrie, who lives in the dorms. As she's about to go, Mercer goes on air and his voice warps. Veronica realizes that it is a taped repeat of an earlier show, meaning he isn't at the radio station and rendering his alibi for previous rapes void. She rushes to the dorm to save Carrie.
At the dorm, Carrie's door opens revealing that Mercer is indeed the rapist. He sits on the bed, explaining to the girl his motives. Being attractive and charming, Mercer "knows" that had he spent an hour chatting her up she would have gladly slept with him. "But that's an hour of my life I would have had to spend listening to you go on and on about unicorns," he says. He hears a sound from the closet and goes to investigate only to find that his victim is stashed inside. Veronica had taken her coat and was hiding on the bed. He realizes this right as she's about to hit him with her taser, and he grabs her arm to deflect some of the damage. Mercer threatens her, and they have a brief scuffle. In the end, Veronica scratches his cheek and stabs him in the leg with a porcelain unicorn before she escapes. She's intercepted by Mercer's accomplice, Moe Flater, her RA. Veronica is oblivious to the partnership, and Moe drugs her before leaving to get Mercer. When she sees a picture of the two boys together in the prison experiment (Mercer is the guard and Moe is the prisoner), she realizes the mistake and manages to hide in the closet, calling her father on Moe's cell phone. In the closet, she finds a hammer and grabs it. She also finds boxes of hair from the victims.
Mercer returns with Moe in tow, and at first, they think that Veronica escaped through the window. Mercer dominates and controls Moe, slapping him for his mistake and calling him "prisoner". The cell phone ringing from the closet alert the duo to Veronica's location, and Mercer orders Moe to get the cell phone away from her. When he attempts to do so, Veronica brings the hammer down on his foot. The injured Moe falls back, howling in pain. Mercer drags Veronica out of the closet before barking more orders at Moe. He tells Moe to give the girl more GHB so she won't remember the night and Moe blubbers that he's out. Mercer still has some in his room, so he insists that Moe watch her while he gets the drugs. Veronica, in the meantime, has found a rape whistle in her pocket and has summoned up the strength to blow it. The conspirators get it away from her almost immediately, but not until after she's made a considerable amount of noise.
Mercer hobbles to his room to grab a vial of GHB, but on the way back he's intercepted by Parker who heard the whistle. Ironically, Veronica questions the rape whistle which was given to her by Parker, skeptical that it would do any good, and would anyone actually help if they heard it. Parker confronts him in the middle of the hallway, insisting he tells her where she is. She puts a hand on his chest to restrain him, and when he tries to remove it she screams "Rape!” calling other dorm residents into the hallway, including Moe. One asks Mercer why he's so beat up, and Moe keeps asking Mercer what they should do bringing more suspicion down on the boy. When he can't fend off the questions, he makes a dash for it, claiming that Parker is insane and that they've all fallen for her act. Moe follows in a hurry, and Parker discovers Veronica in his room.
The boys try to escape, stopping in a hotel. Moe extracts the unicorn horn from Mercer's leg right before Keith Mars enters, wielding a gun with the intent of dragging them back to Neptune. They are put in jail, and the last we see of them is Logan walking menacingly towards them after purposely getting himself arrested. (Spit & Eggs)
In Season 4, Mercer is revealed to still be in prison for more than a decade. Veronica meets with him to discuss a man he may have information on. He initially withholds the information, expressing his frustration in his sentence being significantly longer than other college rapists he’d seen on the news, but he opens up when Veronica inadvertently threatens to interfere with his upcoming parole hearing.