Zack

X5-599, or "Zack", is one of the twelve X5s, who successfully escaped from Manticore.

On the Run for 10 Years
Zack was arrested shortly after the escape from Manticore, though he escaped 4 hours later, and was not seen again. Over those 10 years, he managed to find and hide the other escapees, keeping track of them and refusing to tell each one about the others in case they were captured by enemies. He is highly mistrustful of regular humans and very protective of his siblings, especially Max Guevara.

2019 and On
Zack first appears in Seattle under the alias of Sam, where he manages to obtain a job working at Jam Pony from Normal. After Max manages to track him down and learns that Sam is really Zack, they fight their way out of Zack's apartment as Lydecker had arrived. Zack seeks Max out after Brin is captured by Lydecker's men. Together the pair work with Lydecker to try and rescue her, but Lydecker betrays them. Zack and Max manage to make it out with Brin, but Brin insists that if, she is to live, she must be taken back to Manticore.

Soon after, Lydecker begins placing wanted posters for Max throughout Seattle. Zack returns to take Max away from the city to somewhere where they will be able to hide. When Max learns that Logan Cale has been admitted to the hospital, he allows her to leave after a short fight. He gives himself to the police and Lydecker in order to keep Max safe.

Zack eventually escapes Manticore, but only after accidentally revealing the locations of most of the other '09 escapees in the presence of a listening device. With the help of Eyes Only, Zack and Max manage to alert the others that Lydecker and Manticore is aware of their locations. After learning that Tinga is unable to get away, Zack and Max travel to Portland to help her. Once safely rescuing Tinga, she and Zack travel to Canada.

Zack again returns to Seattle when Tinga's son, Case, is hunted by Lydecker. After Tinga is captured by Brin and others from Manticore, Zack embarks on a mission to find and rescue her. They eventually find her, but she is dead.

In the episode ...And Jesus Brought a Casserole he gives his life to save Max.

It is revealed that Manticore had kept Zack alive by giving him artificial organs after harvesting his real ones for other X5s. After Manticore's destruction Zack is bought on the blackmarket by a gang of Steelheads, who use him as muscle. When he first sees Max he doesn't remember her, because parts of his brain have been replaced. Slowly his memories come back together, he leaves the gang and helps Max and Logan.
 * Some Assembly Required

After trying to kill Logan, Zack's memory was once again wiped from Max having electrocuted him, and he was given a new identity: Adam Thompson, a farm worker.

Following Zack's departure, there has been no more talk about him from Max, or about the other kids that got out in 2009.