Zack was the person Cloud pretended to be for most of Final Fantasy 7. Zack was a former 1st-class Soldier, Aeris' first love, a superior warrior, and an all around nice guy.
He lived with his parents until 10 years ago, when he went to Midgar to become part of Soldier. He became a first class Soldier and met Aeris.
Zack was sent on the Nibelheim mission with Sephiroth and his good friend Cloud. Zack was at the fire of Nibelheim and he did his best to help everyone. At the Nibel Reactor, he tried to stop Sephiroth but failed.
Cloud and him were captured by Hojo and used in the Sephiroth Cloning experiment. Cloud reacted to the Mako and Jenova cells, but due to his training in Soldier and previous exposure to Mako, Zack was unaffected. He made a plan to escape when their meals were brought to them. He carried Cloud's paralyzed body out of the ShinRa Mansion and they managed to get a ride to Midgar in the back of an old man's truck.
Once Zack and Cloud arrived to Midgar, they were attacked by a Group of Soldiers. Cloud was pretty helpless so Zack was forced to fight alone. Zack managed to take out the first few Soldiers, but was shot repeatedly while he was checking on Cloud. Cloud was left for dead and tried to take-over his identity.
Aeris never really got over his death and his parents never found out about his death.
As he demonstrated with Cloud, Zack values his friends and loved ones lives above his own. He is a dangerous enemy and a great friend.