The son of a single parent -- Zack's father ran off as soon he found out that Zack's mother was pregnant -- Zack was raised along with his twin sister Leigh at his grandfather's house in Arlington, right next door to Taryn Maddock and her family. Their mother, Sarah, raised them until the age of six, when she succumbed to cancer and passed away. Thereafter, their grandfather, lifelong Arlington native and retired factory foreman Max Marshall, took over.

Max did the best he could to fill the void in their lives, taking the twins on hunting and fishing trips, trying to help them with their schoolwork and generally spending as much time with them as he could, making sure they never wanted for attention or anything else they needed. Eventually Zack developed his own interests -- as he grew older, he demonstrated a genuine love and something of a knack for chemistry, particularly when it gave him the chance to blow something up, as well as a talent for dealing with computers and electronics. Max couldn't help him as much there, but when Zack's increasingly nerdy reputation made him a target for bullies, he did teach his grandson some rudimentary self-defense and encouraged him to make it known he wouldn't be a victim. Zack and his grandfather remained close through the twins' adolescence, and when Zack left for college, he didn't go far -- choosing Greyson so he could have some independence while staying close to Max.

Compared to his twin sister, Zack has usually been the good child; though they share a somewhat mischievous nature, he plans to act deliberately and after weighing the consequences, and tends toward a slightly more cynical attitude, where Leigh (or Lee, as she often prefers to spell her name) is reckless, impulsive, boy-crazy and easily trusting. It's little wonder that Lee bonded with Taryn's similarly wild twin Rae, while Zack's best friend since childhood has been Taryn herself, who tends to be more cerebral, cautious and shy, and -- while she's not as blatantly geeky as Zack -- certainly at least gets his interests without mocking him too much. Taryn is also the girl Zack's had a crush on since before he really started noticing girls at all, and while she's never noticed and they've both dated other people, his interest in her continues to eat at him. At Greyson, they live across the hall from one another, sharing custody of Blockhead, their senior year project for an MIT science competition, a childlike AI in a small robot body. The tension is growing somewhat thick, and while Zack respects her continued close relationship with Carter Fielding and values their friendship too much to act just yet, the temptation to do so is steadily mounting.