Zack was a member of SOLDIER, and Cloud's best friend, who was, obviously, also in SOLDIER. (But no First Class-that was delusion.) Zack was First Class, and worked alongside Genesis and Angeal for the Shinra, until he ended their rampages. After this, he was branded a traitor by Shinra, and just like Cloud, was used as a specimen by Professor Hojo; he had Mako infused within him. (Not to be mistaken with Sephiroth's JENOVA cell injection.) Him and Cloud both managed to escape the Shinra laboratories, and while Cloud was successful, Zack was shot by Shinra soldiers outside Midgar, and died. (The only real scene about him you see in FFVII, after visiting his parents in Gongaga.) The death of his friend, coupled with the experimental abortion within him is believed to have made Cloud lose part of his sanity, though this may be attributed to other factors, such as desire...Cloud took Zack's personality and made it his own, based on stories Zack told him, what he knew of him and what he told him a First Class SOLDIER was like, and what his duty was. Zack does not exist during the entirety of FFVII outside of Cloud's split personality disorder, and what Cloud portrays of him is not true-aside from the Neibelheim incident, this is where it gets complicated. Sephiroth, Zack and Cloud were to investigate the old Nibelheim reactor, in order to fix it up...(A ploy by Hojo to actually accelerate and deploy the dormant JENOVA cells within Sephiroth.) When Sephiroth freaks out, Cloud claims to have defeated him, but everything you see about Cloud in this flashback was actually Zack's doing; Cloud was the rookie soldier always accompanying him; his admiration of Sephiroth was always silent. Zack defeats Sephiroth, while Cloud remains outside, and saves Tifa from her fatal navel wound, if you recall the flashback. Zack throws him within the Mako beneath the bridge, Sephiroth survives this because of his infusion. (?? not sure about that Sephiroth thing about survivng, but who cares, has nothing to do with Zack lol.) So, that's all I can say for that, I hope it helps a little. But yeah, mostly he was Cloud's friend, Aerith's first boyfriend, worked with Angeal and Genesis, betrayed his employers and was then used as an experiment by Hojo, tried to make a break for it and died in the process-in FFVII, he basically just served to fashion Cloud's problems, and move the story onward, but he became popular enough in the years to come...don't they say a lot about him prior to the FFVII events in CC though?