Zack is the only of Max Thompson's friends to not be present when he discovers an old mirror stored away in a hidden room of his attic. When Max discovers the light overhanging the mirror has the ability to make him invisible, he knows Zack will be the perfect target to scare with it.
Inviting him over, Max, along with his brother, Lefty, take Zack up to the attic, where they shock him with their discovery. Immediately, Zack is interested in the mirror and wants to try it out, but Max is reluctant to let him. When Max's friends, Erin and April show up to check out the mirror, Zack decides to have a competition over who can stay invisible the longest.
While Zack's obsession with the mirror grows, so does Max's concerns over the true nature of it. Whenever he goes invisible, Max feels ill, like he's being pulled away to an unknown location.
When Zack goes invisible for a long time, he comes back different. His haircut seems flipped, and his behavior is out of character. He and Erin urge Max to use the mirror once more, but he is hesitant. In response, Zack and Erin forcefully push Max towards the mirror and turn on the light. When he goes invisible this time, he finds himself confronted by his reflection, who reveals that Zack and Erin have been replaced.
Max's reflection attempts to switch with him, but Max manages to get out, and Lefty breaks the mirror with a softball, setting the real Zack and Erin free, and destroying their evil reflections.
Max describes Zack as an intense person. He loves competition, and hates to lose, getting really upset if he ever does. His love for competition eventually leads to an obsession with staying invisible longer than his friends, and later, it leads to him being replaced by his mirror doppleganger.
The book describes Zack as having a serious-looking face with tiny blue eyes. He has black hair, shaved short on the left side, and the rest is long and combed straight. When his reflection takes over, the haircut is switched.