Zack is played by William Gregory Lee, who has also appeared in some episodes of Xena: Warrior Princess and Beauty and the Beast.

Out of all the children who escaped from Manticore, Zack (designation 599) is the leader, and the one who basically instigated the escape. It's been made pretty clear that he has kept an eye on all twelve of the X-5s, seeing them as his responsibility even though they all escaped.

Zack is over-protective of his brothers and sisters, and this puts him at odds with Max a number of times in the episodes he shows up in. Specifically, this happens when Max is accused of a murder he committed, and he fights her to try and stop her from going back.

At the very end of season one, Zack shows his care and loyalty to his siblings in a huge way when he shoots himself in the head so that his heart can be given to Max and save her life. Zack doesn't die, though, and is basically put through conditioning to make him think that he and Max were together - and that Logan caused them to break up.

In the final episode that he's seen in, Zack is given a new identity and a new family, with no memory of Max or any of the other X-5s...