Final Fantasy XV characters wear black because it’s the official state uniform, evidently.
(Presumably the open shirt is for purposes of distraction during battle.)
All joking aside, game director Hajiime Tabata detailed why the design decision was made to have all the main characters in FFXV wear all-black in an interesting forum post.
“The clothing worn by the four main characters is the official battle gear of the Kingdom of Lucis,” he said. “Noctis is a member of the royal family and the other three characters are parts of various Lucian national organizations and they are each wearing the specific battle dress relevant to their individual stations.”
Tabata went on to explain that black is a special color in the Kingdom of Lucis, where the game takes place. “Because of this, the attire of the royal family and those in occupations closely tied to them are unified by having black as a base coloring. This part of the lore is the reason why the outfits of all four main characters are black.”
And, perhaps most interesting: “For the finished game we also intend on having their clothing having effects in battle and not just being a part of the world and its background. So watch out for that too!” This hearkens back to Final Fantasy XIII: Lightning Returns, where players could customize Lightning’s skills and techniques by donning different outfits.
Final Fantasy XV will be available sometime in 2016 for PlayStation 4 and Xbox One. 1 upvote