Zack Fair (Japanese: ザックス・フェア, Hepburn: Zakkusu Fea) is a character in the Final Fantasy role-playing video game series by Square Enix (originally Square). He is first introduced as a non-player character in Final Fantasy VII (1997) and later appears in the Compilation of Final Fantasy VII works Before Crisis, Last Order, Advent Children and, most prominently, Crisis Core, a prequel to the original game which focuses on him and expands on his backstory.

Although Zack died before the events of Final Fantasy VII, is rarely mentioned, and only appears late into the story in flashbacks, he is a crucial part of the game's story, as following traumatic events involving them and Sephiroth, Cloud Strife unknowingly confused parts of his past with Zack's. While Cloud originally believes himself to be a member of the paramilitary organization SOLDIER, the military wing of the megacorporation Shinra, this story was in fact that of Zack, with Cloud being his friend and a regular soldier. Zack died in the weeks leading up to Final Fantasy VII's opening, sacrificing himself to protect Cloud from Shinra's army after they escaped from imprisonment; a mentally shaken Cloud's memories of the events were heavily affected, and he forgot Zack entirely.

Zack was not originally part of Final Fantasy VII's story during development, but scenario writer Kazushige Nojima wanted to bring a sense of mystery to the title, and created the character to help complicate Cloud's backstory. He was designed by Tetsuya Nomura, and his last name is derived from "fair weather", to contrast with Cloud's last name, Strife. With Zack's conceptual backstory in place for Final Fantasy VII, the staff decided to use Compilation of Final Fantasy VII to expand upon his character. Zack is voiced by Kenichi Suzumura in Japanese and Rick Gomez in English prior to Final Fantasy VII Remake, in which he is voiced by Caleb Pierce. Suzumura was chosen specifically by Nomura for his voice, and was given the role without an audition. Western critics have praised Zack's character, commenting on his development since Final Fantasy VII.

Zack did not exist in the original scenario of Final Fantasy VII, but was created when scenario writer Kazushige Nojima decided to add some mystery to the plot, most notably in relation to Cloud Strife's background. Nojima had always planned for Cloud's memories of his life to be proven false as the game went on, but he had not decided on how to implement this until he hit on the character of Zack. Nojima also used Zack to link Cloud and Aerith Gainsborough, as Zack was her first love and Cloud reminds her of him, thus creating an emotional connection between herself and Cloud. Originally, the role of her first love was to have been fulfilled by the game's antagonist Sephiroth as suggested by Nomora. As the game continued development, Nojima worked out the mysteries regarding Zack and Cloud, which led to some scenes needing revision. Director Yoshinori Kitase was surprised by the revelation of Cloud's and Zack's connection, as until the later stages of development, even he did not know about Zack. Character designer Tetsuya Nomura got the request to design Zack when Final Fantasy VII was reaching the end of development. Prior to the late addition of Zack, Nojima had asked the staff to add details to some scenes to hint at his existence, despite not revealing Zack's existence to the staff until later. The opportunity to write Crisis Core would gave Nojima the ability to expand on Zack's character and create a personality for him. Although Zack is linked with Cloud due to multiple parallels, Nojima stated that the personality of Zack was based on Final Fantasy X protagonist Tidus, as both are rarely bothered by their own thoughts and are generally cheerful.

For Final Fantasy VII: Advent Children, Zack made only a couple of brief appearances, and as such, was not difficult to animate; the team had also acquired his design early in production, allowing modeling of his character to be taken care of. Nomura had wanted Zack to have a "nice, upbeat voice", which influenced his decision to cast Kenichi Suzumura. Beforehand, Nomura had had dinner with Suzumura, where he had decided that "at that point [Nomura] wanted him to be in one of his projects if the opportunity ever presented itself". Suzumura was offered the role without an audition. Nomura explained that, because Zack had been chosen to be the lead in Last Order: Final Fantasy VII, he needed "someone who could handle [the] role well". The staff used Last Order as an opportunity to portray Zack "properly" as a "handsome, light-hearted man [who] was in everyone's memory". In English, Zack is voiced by Rick Gomez.

In an interview with IGN whilst promoting Crisis Core, Yoshinori Kitase explained that when the original game was created, Zack was a minor character, although Nomura had created art design, and Nojima had created a basic concept of Zack's story. Kitase further explained that "you could say that the idea [for the storyline of Crisis Core] has been cooking for 10 years". Before Crisis Core began development, the staff had planned to create a PlayStation Portable port of Before Crisis: Final Fantasy VII, but soon changed their minds to create a game that focused on Zack, whose fate was predetermined; fans knew how the game was going to end. Throughout the game, the staff decided to use a blue sky in cutscenes to represent Zack, while other features in such scenes are meant to symbolize his connections. Several of Zack's actions from the game were also designed so as to augment the similarities that Aerith finds between him and Cloud in Final Fantasy VII.

Due to Zack initially appearing as a younger person than in previous games, Suzumura was asked to portray Zack in a fashion that shows him to be naive for his character arc to be more noticeable, as he becomes more mature throughout the narrative. He wears the SOLDIER 1st class uniform, consisting of a black, sleeveless turtleneck, black boots, and armor. In Crisis Core, Zack has two outfits; his Final Fantasy VII outfit and a different outfit worn during the start of the game, which he changes after fighting Angeal Hewley. Originally, Zack was meant to wear red clothing until Tetsuya Nomura drew his black outfit. He was made as a conventional heroic character in contrast to Cloud, who was written as an unlikely hero. There was an attempt to make Zack's and Aerith's interactions appealing in Crisis Core. Zack's full name was first revealed in an article in Dengeki PlayStation. Nomura stated that Zack's name was derived from 'fair weather' and specifically chosen because it contrasted with Cloud Strife's name. Zack and Cloud's connection was also meant to be expanded upon near the game's ending, with both of them planning to flee to Midgar, but due to limitations in the console's hardware, these scenes could not be implemented, and instead, they decided to focus on Zack's role as a warrior. The game's remaster, Crisis Core Reunion, was developed as part of the Final Fantasy VII Remake project and acts as a prequel to the main trilogy designed to familiarize players with the story of Zack ahead of his appearance in Final Fantasy VII Rebirth, where he would play a more significant role.

Zack had a small role in the original Final Fantasy VII. He is first mentioned by name in Gongaga, his hometown, where his parents do not know what happened to him after he left to join SOLDIER, and are worried for his safety after not hearing from him for years. It is at this point that Aerith explains Zack was her first love. Cloud later realizes that some of his memories and aspects of his personality were actually Zack's, and not his own. Flashbacks reveal that both Zack and Cloud battled Sephiroth after he burned down the town of Nibelheim upon discovering he was born from a scientific experiment. After Cloud defeated Sephiroth, both Zack and Cloud were taken to be used in experiments by Shinra. Eventually, Zack awoke and was able to escape with a semi-conscious Cloud to the city of Midgar, but on the edge of the city, Shinra troops gunned him down. While the flashback showing Zack's and Cloud's escape is optional in the North American and European releases of Final Fantasy VII and the Japanese International version, it was originally planned to be shown once Cloud discovered the results of Shinra's experiments.