Just as a Modern Day Adonis
What many people don't realize is Zack was a last minute edition to Final Fantasy VII. While the Square Soft staff had decided early on that Cloud's personality and even his history would be a lie, they didn't consider where it had originated from. Therefore, Zack's appearance may have been based on Cloud's (who was designed first) to show how in the game, Cloud had rebuilt his own mind in the image of Zack.
Zack was one of the last characters Nomura designed and like Cloud; Zack has crazy, messy hair known for its gravity defying spikes. Unlike Cloud however Zack's hair is a deep brown, which may come off as black in some of the official artwork. Zack has a long face with a strong jawline; his face is often marred with a small x-shaped scar on his cheek that he obtained during his final battle against Angeal. Zack always carries himself like a proper member of the military, unlike Cloud, who leaves his shoulders slack, like a beaten dog. Zack wears a navy blue SOLDIER uniform which is standard among First Class members, what makes Zack's uniform unique is that he wore it even as a Second Class SOLDIER, instead of the regulation purple or blue uniforms.
While many of Zack's features, including the above, are static throughout the Compilation, other features have been changed to suit the needs of the Square Enix staff members. For example, many fans were under the impression that his eyes were purple, which is supported by his profile picture in Square's Ehrgeiz: God Bless the Ring, fighting game. In the compilation, it was made clear that his eyes are supposed to be blue. Of course, what shade has never been made clear, if fact in some pictures they seem more of a teal green rather than blue at all. In most pictures however, they are somewhere between cerulean and sky blue. Another changing feature is Zack's scar. The scar, while present in the later part of Crisis Core and in Advent Children Complete, was not included in the original game, Before Crisis, Last Order, or the original cut of Advent Children. The addition was most likely included to stress the importance of Angeal and Zack's relationship, and the guilt Zack felt over his death, however, it does appear, although somewhat differently, in Ehrgeiz which leads me to believe that it may be the actual origin of Zack's scar.