Zack left his hometown of Gongaga at the age of 14 to join the ranks of ShinRa Electric Power Company's SOLDIER forces in the city of Midgar. After spending some time as a grunt in ShinRa's common military forces under the command of Heidegger, Zack successfully entered SOLDIER with a 3rd Class ranking. He was placed under the mentorship of 1st Class SOLDIER Angeal Hewley, and quickly earned promotion to 2nd Class, whereupon he made friends with fellow 2nd Class Kunsel and worked under the command of Lazard Deusericus.
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At age 16, Zack single-handedly headed the takeover of Fort Tamblin, definitively ending the war with Wutai despite a mass desertion of 2nd and 3rd class SOLDIER members under the leadership of 1st Class SOLDIER Genesis Rhapsodos, Angeal's childhood friend. During the siege, Angeal disappeared and Zack was attacked by a swarm of copies sharing Genesis' appearance. Seeking the missing SOLDIERS, ShinRa subsequently ordered Zack and Turk leader Tseng to Angeal and Genesis's hometown of Banora to question Genesis' parents, only for both men to be attacked by more Genesis copies and the original Genesis himself. When Zack attempted to engage the 1st Class, Angeal appeared and stopped him, allowing Genesis to escape. Chasing after the two, Zack stumbled upon the corpse of Angeal's mother and was attacked in earnest by Genesis, who summoned Bahamut to the fight. Upon its defeat Genesis cryptically stated that he and Angeal were monsters, unfurled a large black wing from his shoulder, and flew off. Tseng and Zack were subsequently left to evacuate Banora as it was bombed by ShinRa to hide evidence of Genesis' actions. With Genesis and Angeal declared killed in action, Zack was returned to Midgar, where he was promoted by Lazard to 1st Class.
Shortly afterward, another swarm of Genesis copies attacked Midgar, and during the attack Zack and Sephiroth discovered the lab of Professor Hollander, a ShinRa scientist with evidence of having helped create the copies. In pursuing Hollander Zack made the acquaintance of the Turk Cissnei and encountered Angeal yet again, who defended Hollander, displayed two white wings and claimed that the professor and Genesis would help him find a cure for his mutation. Refusing to fight his mentor, Zack was thrown down into the Sector 5 church, where he met Aeris and managed to befriend her before rejoining the fight.
By the time Genesis' attack on the city was defeated, Angeal had decided to return to ShinRa's side, and Genesis had disappeared. Only a few months later, Zack was assigned to a mission in Modeoheim along with Tseng and Cloud Strife (whom he was meeting for the first time). Upon investigating a suspicious reactor along the way, Zack found Hollander and Genesis in an argument and engaged Genesis in a fight. Defeated, injured and deteriorating, Genesis defied capture by jumping into the reactor pit. Zack pressed on to find Hollander and Angeal in Modeoheim, where he discovered that Angeal was actually Hollander's son and one of ShinRa's many human experiments. Also deteriorating and devoid of purpose, Angeal was overcome by his mutation, changing into a monstrous form and finally forcing Zack to fight him to the death. At the end of the battle, Angeal handed his buster sword and his legacy onto Zack with the last of his strength, then quietly passed away. Stricken, Zack took Hollander in custody back to ShinRa and was then put on standby in HQ.
After spending about two years on a forced "vacation", during which he was kept close to HQ and formed a stronger friendship with Sephiroth and the Turks, Zack was called to Junon only to find that Genesis clones were attacking the city and had broken Hollander out of custody. After failing to recapture Hollander, Zack returned to Midgar where more clones were attacking, and arrived in time to witness an Angeal clone protecting Aeris from an attack before taking off. The city was secured once again, but there was news director Lazard had deserted the company prior to the attack, leaving SOLDIER under Heidegger's command.
After a few weeks of quiet, Zack was next assigned to a mission in Cloud's hometown of Nibelheim, along with Cloud and Sephiroth. There he met Tifa, who guided the group up the mountain to the reactor, where and the group discovered a pod with a piece of JENOVA inside. Upon return Sephiroth went to investigate the local ShinRa mansion for clues about what JENOVA was. The information about his origins that he found there drove him mad, further amplified by the JENOVA cells in his own body coming awake and starting to speak to him inside his head. No longer in his right mind, Sephiroth razed Nibelheim to the ground before fighting Zack, Cloud, and Tifa inside the reactor. All three were grievously injured before Cloud was able to defeat Sephiroth by throwing him into the reactor core. Incapacitated, Zack and Cloud were then captured by ShinRa and given to Professor Hojo, who held them captive in the mansion's basement while performing medical and scientific experiments on them. During this time period, Nibelheim was rebuilt and repopulated with ShinRa employees to cover up the incident.
After four years under Hojo's control, Zack eventually managed to break his way out of the mansion lab, dressing himself and the now-comatose Cloud in SOLDIER uniforms and attempting to escape back to Midgar. On the way he was intercepted by Cissnei, as the Turks had been sent by ShinRa to retrieve or exterminate the "escaped experiments". Cissnei decided to aid them in their escape instead, throwing ShinRa off the trail and giving Zack a motorcycle so that he might outrun the army more easily. Shortly afterwards, however, Zack was set upon by Genesis, who had survived his drop into the reactor. Unable to properly fight while protecting Cloud, Zack managed to escape the conflict and continued onwards only to stumble upon his hometown of Gongaga, which had been heavily damaged by a reactor explosion triggered by the original AVALANCHE. Sick with worry for his family, he left Cloud on the outskirts of town and entered only to be intercepted again by Cissnei, who filled him in on how his parents were doing and informed him that Angeal had been spotted in the town. Chasing after Angeal, Zack encountered Hollander instead, who had taken on the deteriorated appearance of Genesis and was after Cloud for untainted the JENOVA cells that Hojo had injected the non-SOLDIER with. Hollander was defeated with the aid of "Angeal", who turned out to be Lazard, infected with Angeal's mutated cells to the point where he had gained the 1st Class's appearance and was beginning to deteriorate as well.
From Gongaga Lazard and Zack continued on together to the town of Banora, where Zack encountered the original Genesis yet again, with the 1st Class absorbing what he refers to as the Goddess Materia before engaging Zack in a fight. Upon defeat Genesis returned to normal, cured of both his mutation and his genetic degradation. Now human, he challenged Zack to one final duel before being defeated for good. Leaving Genesis behind, Zack and Lazard moved on with Cloud in town.
During the altercation the group was found by both ShinRa soldiers and the Angeal clone that had rescued Aeris, and Lazard and the clone both died protecting Cloud from extermination. Anguished at the losses, Zack took Cloud and continued onto Midgar, leaving Genesis behind. But where his original counterpart hitched a ride on a truck to cross the wastelands, Zack made a split-second decision to skirt the wastelands and steal a new vehicle, not wanting to put any civilians at risk or make it any easier for the army to track them. Due to this single, tiny change in plans, Zack was able to successfully evade the army long enough for the Turks to lead the soldiers further off their trail, enabling him to reach Midgar.
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Zack and a recovering Cloud reached Midgar shortly after the terrorist group AVALANCHE had blown up the city's first mako reactor, and were able to sneak into the slums under the cover of all the confusion. Relying on old civilian friends and contacts, Zack was directed to the Seventh Heaven bar in Sector 7, where he was surprised to find Tifa working together with Barret as leaders of the AVALANCHE faction. Tifa offered to hide them from ShinRa after hearing their story, and Barret tried to convince both men to join AVALANCHE's cause. While he agreed with their motives, Zack couldn't condone the loss of life that was caused by their methods, and got into a huge argument with Barret over the plans to detonate a second reactor. When Barret protested that they didn't have any other options, Zack volunteered to infiltrate ShinRa HQ and look for another way to shut down the reactor. Barret agreed, and Zack did as he promised. While in HQ, however, he came across experiment Red XIII, a Cosmo Canyon beast by the name of Nanaki. Zack took the time to free Nanaki from Hojo's lab, but in doing so, raised the alert among the guards and was forced to stay hidden in the building and wait for a chance to sneak out again.
Tired of waiting and worried about ShinRa discovery, Barret decided to go ahead with the reactor detonation. Despite Cloud's misgivings, Tifa convinced him to come help, and the three went with Biggs, Wedge, and Jessie to the number 5 reactor. Once there, the team accidentally tipped ShinRa off to their presence, and the ensuing red alert in HQ gave Zack and Nanaki the chance they needed to escape and make their way to the reactor. They arrived just in time to help the rest of AVALANCHE fight President ShinRa's Airbuster drone, but the drone self-destructed upon defeat, destroying the walkway and dropping the entire group down into the Sector 5 slums and the church below. There, Zack was
Not expecting to find Zack (let alone all of AVALANCHE) there, the Turk sent the ShinRa lackeys he was with off for 'reinforcements', and used the time gained to warn Zack about attracting too much of ShinRa's attention before letting them all escape. With the group worried about Reno's vague warnings but needing more information, Aeris suggested that she and Tifa infiltrate Don Corneo's to look for details. Cloud and Zack refused to let the girls go alone, so they got disguised as women and all four went to infiltrate while Barret and the others went back to guard the bar. They discovered that ShinRa was planning to drop the upper plate on Sector 7, but were then tricked by the Don and dropped into the labyrinthine sewers beneath his mansion. By the time they finally get out and back, the Turks had already dropped the plate, killing hundreds of people who lived in the sector below — including Biggs, Wedge, and Jessie. Barret and his daughter Marlene were the only AVALANCHE members to escape alive.
Everyone regrouped at the house of Aeris' mother, Elmyra, intending to lie low while they came up with a new plan. A few short days later, however, Zack and Cloud were disturbed by hearing the voice of Sephiroth in their heads, and followed it to ShinRa HQ just in time to discover that Sephiroth was alive, had assassinated the President, and that Rufus ShinRa had quickly succeeded him. Ruthless and eager to use AVALANCHE as a scapegoat for public persecution, Rufus attempted to have the team arrested on the spot, and his efforts forced them all to flee the city and take refuge in Kalm. Yuffie Kisaragi, Vincent Valentine, and Cait Sith joined the group, and Zack convinced them all of the need to find and stop Sephiroth.
AVALANCHE spent some time futilely chasing after Sephiroth, getting entagled in a number of events along the way, such as getting thrown into prison at the Gold Saucer. Eventually they found their way to Cosmo Canyon, Nanaki's home, and met the elderly Bugenhagen, who explained to all of them the nature of mako and the Lifestream, and just how ShinRa's reactor methods were destroying the planet. Zack and Nanaki went with Bugenhagen to confront Nanaki's past, after which he definitively decided to join the team. From there, AVALANCHE decided to go to Nibelheim to search for clues to Sephiroth's whereabouts, where they found Sephiroth himself and engaged him in battle. Sephiroth escaped, declaring his intention for a Reunion at the North Crater, and the team followed him north, meeting Cid Highwind and acquiring the Tiny Bronco.
Through Cid, and Zack's loyal contacts in SOLDIER, the team discovered that Rufus and Sephiroth were both headed for the Temple of the Ancients, and went to acquire the Keystone necessary to gain access to the Temple. In getting it Zack revealed that he'd identified Cait Sith as a ShinRa spy, but allowed him to stay with the group on the condition that he lead them to the Temple. Once there, they discovered a grievously wounded Tseng, and Zack appealed to Vincent's history as a Turk to convince him to get Tseng to safety while the rest of them continued after Sephiroth. Further in the Temple, they found Sephiroth and discovered he was trying to use the Black Materia to summon Meteor in an attempt to make himself a god. Zack fought him, but was critically distracted when Sephiroth tried to take control of Cloud's mind during the battle, and Sephiroth escaped once again. Cait Sith, being a construct, sacrificed himself so they could acquire the Black Materia and keep it away from Sephiroth.
Aeris decided to see if she could use her powers as a Cetra to destroy the Black Materia entirely, and so, after reuniting with a rebuilt Cait Sith, the group travelled to the City of the Ancients to see if there were any clues. While they were there, Sephiroth invaded Cloud's mind once again, compelling him to try and kill Aeris before she could find a way to destroy the Materia. Zack defended Aeris from Cloud's attack, but while he was engaged in fending Cloud off, Sephiroth dropped from the sky and ran Aeris through, instantly killing her. Enraged, Zack was distracted long enough for both Cloud and Sephiroth to stab him through his left shoulder from opposing sides. Despite this debilitating wound, Zack knocked Cloud unconscious and viciously attacked Sephiroth, finally succeeding in killing the general and former best friend.
But the battle wasn't over, as with Sephiroth's death JENOVA immediately possessed Cloud instead, and exerted enough mental pressure on Zack to cause him to collapse. With Aeris dead, Zack incapacitated, and Cloud now fighting with Sephiroth's strength and abilities, the rest of AVALANCHE was forced to escape with Zack and retreat back to Bone Village. Demoralized, their leader emotionally unresponsive, they tentatively decided to continue north again in an attempt to find Cloud. They travelled all the way to Icicle Inn, where they discovered the research of Professor Gast, Aeris' father. Zack finally regained some of his old self when he realized the research provided clues as to a way to save Cloud — if they could kill JENOVA, he might be returned to his old self. Before they left, Elena of the Turks showed up, grateful for their rescue of Tseng, to inform them that Rufus was heading to the North Crater to examine it. Zack got her to carry a message back to Tseng requesting information on where ShinRa had hidden the different parts of JENOVA, and then split the team up into two parts. One half, containing Nanaki, stayed behind with the Black Materia to guard it, while Zack led the second half onwards to confront Cloud in the crater.
Cloud was attempting but failing to combat JENOVA's control, inundated with Sephiroth's memories and skills and quickly losing grip on both his sanity and sense of identity. Zack attempted to talk him out of JENOVA's influence but was foiled by the arrival of Professor Hojo, whose clinical analysis of Cloud and Sephiroth's purpose finally pushed Cloud over the deep end, forcing Zack and the others to battle him. During the battle, JENOVA asserted control over Nanaki through the JENOVA cells in his system and coerced him to bring the Black Materia to Cloud, who took off as soon as it was in his possession. Before the team could react, the chaos woke up the ancient guardian WEAPON, which caused the cave to collapse and forced everyone to evacuate or be crushed.
Tifa, Barret, Nanaki, and Yuffie were captured by ShinRa in the process of escaping, and brought to Junon by Scarlet to be set up for public execution. Tifa and Yuffie were already in the gas chamber when WEAPON suddenly attacked the city, giving Cait Sith the chance to turn on ShinRa and free Barret and Nanaki. Yuffie lockpicked their way out of the gas chamber, and both girls escaped out onto the cannon where they were confronted by Scarlet. After a brief fight, they were picked up by Zack in the others in a stolen ShinRa airship. Once everyone was safe on board, Cait Sith informed them all about ShinRa's new plans to use SOLDIER to find and kill Cloud before he can summon Meteor, and at the same time the Turks rang Zack up with info on JENOVA parts. Considering the destruction of JENOVA more pressing, Zack contacted his friends in SOLDIER and begged them to try and hold Cloud off rather than kill him, buying them some time.
They'd managed to destroy roughly half of JENOVA's parts before Kunsel contacted them back, telling them to come to the crater where the Temple of Ancients used to be. AVALANCHE arrived there to find all of the SOLDIERs under the control of Cloud/JENOVA, with many of them suffering mutations, including Cloud. Surrounded, they were forced to fight and destroy the SOLDIERs, with much grief on Zack's part. They won, resulting in the decimation of the last remnants of SOLDIER — but not before the altercation had given Cloud enough time to summon Meteor and escape yet again. The group retreated to Mideel, and Cait Sith told them of another change in ShinRa's plans: now Rufus intended to load a rocket with Huge Materia and fly it into Meteor in an attempt to destroy it. Zack decided to let ShinRa try the rocket while the rest of them went to see Bugenhagen, but at that moment Meteor's approach awoke Ultimate WEAPON, triggering a massive earthquake in the area. Zack was cast into the Lifestream where it ran close to the Planet's surface, leaving the others to assume he was dead. Distraught, the team divided into two: Tifa, Nanaki, Yuffie, and Barret decided to continue on to Bugenhagen as Zack had suggested, while Cid, Vincent, and Cait Sith went to investigate ShinRa's rocket and the Huge Materia.
In the Lifestream Zack was alive, but suffering through a mental revisitation of all the deaths he'd caused and witnessed. Assaulted by his guilt and emotional pain, he was considering simply letting himself die and allowing AVALANCHE to finish the quest without him when he was visited by Aeris' spirit. She encouraged him to keep trying, citing all the people that were depending on him and waiting for him to come back, and told him that he was going the right path in destroying JENOVA. If he could defeat her, Aeris would be free to summon Holy to stop Meteor. Aeris also told him where to find her White Materia, instructing him to hold onto it and have faith in Cloud. Promising that she'd always love him and be with thim, she then threw Zack back out of the Lifestream.
Zack retrieved the White Materia and then reunited with the others in Cosmo Canyon, where he learned that while he'd been in the Lifestream, ShinRa had successfully launched the rocket into Meteor. Cid's team declared that the attempt had only destroyed a little less than half of Meteor, and that the rest was still heading for the Planet. Additionally, ShinRa had discovered Cloud had returned to the North Crater, and had had Junon's Sister Ray cannon moved to Midgar with the intention of destroying Cloud and WEAPON in one go. Hojo was also in Midgar, personally guarding the final remaining pieces of JENOVA.
AVALANCHE reached Midgar just as WEAPON began its assault on the city, and used the airship's guns to assist the Sister Ray in destroying WEAPON. There was a trade volley wherein the Sister Ray destroyed both WEAPON and its protective barrier, but not before WEAPON fired several shots that destroyed part of the city, causing Rufus to
At the Crater, half of the team stayed at the entrance to stave off the swarms of monsters that were being attracted by JENOVA's presence, and the other half went on to battle Cloud. He had fused with the last two JENOVA pieces to become a large and powerful monster, almost utterly devoid of the person he had once been. The group attempted to fight him, but between his overwhelming power and their reluctance to kill him, Cloud clearly had the upper hand. He was also clearly tormented by JENOVA, speaking out loud to her and alternating between ranting and silence, and Zack realized that JENOVA was also attempting to overwhelm his own mind like she had when she had first possessed Cloud — but something was stopping her. He noticed that Aeris' White Materia, which he had been carrying with him, was glowing, and purely on instinct he threw it at Cloud. Cloud caught it out of reflex, and almost instantly the Materia's touch forced the JENOVA pieces back out of his body, leaving him mutated but mostly human and unconscious. Zack and the others defeated the two JENOVA pieces after a ferocious battle, finally freeing Cloud of her influence and allowing Holy to come forth. As the power of Holy began to fill the crater, everyone escaped with Cloud to the Highwind, and watched as Holy obliterated the last remains of Meteor — Aeris' last gift from the Lifestream.