A SOLDIER for Shinra, and Cloud’s comrade. He was a considerate man who valued his allies, and was best friends with Cloud and Sephiroth. He participated in many missions for SOLDIER and served the company well, but during the Nibelheim incident he suffered serious injuries, and was confined in the Shinra Manor as a test subject for Hojo. He narrowly manages to escape with Cloud, but with Midgar right before him, Zack was killed. In a state of heavy mako poisoning at the time, Cloud superimposes Zack, who had died to save him, over his own personality.
Even after defeating Sephiroth and regaining his true self, images and memories of Zack turned to a sense of guilt which continued to torment Cloud. Suffering from this, images of Zack appear in his consciousness time and again. Zack will later become one of the driving forces to help Cloud move forward on his own feet.
As Cloud put an end to Sephiroth and broke that curse, Zack was there by his side, seeing him through.
Compilation of FFVII for Zack
Before Crisis -FFVII-
Zack, during his days as a young SOLDIER, appears in chapter 8, “Light Unyielding to the Dark.” In this mission he fights alongside the main character, one of the Turks.
Crisis Core -FFVII-
In “CC,” Zack is the lead character. His past is completely revealed, including his missions during his time in SOLDIER, his friendship with Sephiroth, his first love with Aerith, and his meeting with Cloud. And of course you can see the whole story of the events leading to the start of FFVII, from the Nibelhiem Incident to his heroic end after escaping.