Zack's Story ~Crisis Core: Final Fantasy VII~
"Hey, would you say I became a hero?" -- Zack.
Zack was born in the town of Gongaga, where he lived until he left to join Shinra's elite militant force, SOLDIER. He eventually fell in love with Aeris Gainsborough after falling into the slums. While on the hunt for the rogue SOLDIER Genesis, Zack is promoted to SOLDIER 1st Class a short time before defeating his former mentor and original owner of the Buster Sword, Angeal, who passes the sword onto him while dying. Soon after, Zack went to Nibelheim to investigate the nearby Mako Reactor which had been malfunctioning. However, during the investigation, Sephiroth discovered the truth behind his origins and went on a rampage, destroying the Nibelheim village. Zack confronted Sephiroth in a duel only to be defeated. Following the battle, both he and Cloud were used by Hojo as test specimens. However, Zack managed to escape after four years and carried the incapacitated Cloud with him to freedom. The two were spotted riding the back of a truck headed to Midgar, with Zack putting Cloud in hiding as he fought the numerous Shinra soldiers. After fighting several Shinra infantrymen and being shot multiple times, Zack dies with Cloud at his side. It is after this that Cloud takes Zack's words of him being his "living legacy" seriously, adopting Zack's personality traits, memories, even habitual gestures, as his own.
Role in Final Fantasy VII
Zack's existance is initially unknown to Cloud due to the mako poisioning Cloud suffered from the experiments in Nibelheim. Throughout much of the adventure, Cloud's personality, actions, and even memories replicate Zack's without any knowledge that he exists. However, when Sephiroth confronts Cloud and tells him that not only is he not Zack, he is actually a failed clone of Sephiroth. When Cloud becomes incapacitated by another poisoning of mako, he goes through sequences in his mind where he becomes aware of Zack's existence, as well as his true history. Cloud learns that Zack saved him from the experiments in Nibelheim, and sacrificed his life in order to protect Cloud from the Shinra Military. Cloud accepts this revelation and moves forward to continue his quest to stop Sephiroth from summoning Meteor.
Role in Final Fantasy VII: Advent Children
In Final Fantasy VII: Advent Children, Cloud is shown visiting Zack's Buster Sword on a cliff outside of Midgar, the location where he was killed by the Shinra Military. At the end of the movie, Zack is shown standing at the doorway of the Sector 5 Slum's Church with Aerith, letting Cloud know that everything is good now.
Zack plays a slightly larger role in the extended cut of the film known as Advent Children Complete. During the final battle with Sephiroth, which has been revised from its original version, Zack appears just as it seems that Cloud is about to lose. He asks Cloud if he needs any help before assuring him that he can win, because "you've beaten him before. This should be cinch!" Over the course of the movie, Cloud is seen grieving as much for Zack as he had for Aerith in the 2006 release.