Concept and creation
Zack did not exist in the original scenario of Final Fantasy VII, but was created only 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 Strife'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 Aerith had seen something of Zack in Cloud. Zack was the first person Aerith liked, thus creating an emotional connection between herself and Cloud, because he reminds her of him. Originally, the role of her first love was to have been fulfilled by the game's antagonist Sephiroth. As the game continued in its development, Nojima worked out the mysteries regarding Zack and Cloud, which led to some of the scenes in the game 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 so as to give clues about him, despite the fact that he did not reveal to the staff Zack's existence until later.
Zack appears as a young man with spiky black hair, standing 185 cm (6 ft 1 in) tall. He wears the SOLDIER 1st class uniform, consisting of a black, sleeveless turtleneck, black boots, and armor. In Crisis Core, Zack has two attires; his Final Fantasy VII outfit and a different outfit worn during the start of the game, which he changes after fighting Angeal Hewley. 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.
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 rather 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 already 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. A number 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. 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. However, 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.