Final Fantasy XV may have a bit of a familiar look to the cities, mainly because Square Enix tried to make them an amalgam of real world places. However, the characters were left to find their own story, and were not based on anyone in the real world, with the exception of Prince Noctis.
Lead Writer Dan Inoue said during a FF15 panel at GDC (via Polygon) that Noctis could have easily turned into a generic RPG emo kid, but he didn't want that. While working on the character with the Japanese actor voicing the character, Inoue said he struck upon the idea of Kurt Cobain, the troubled lead of the band Nirvana from the late 1980s, as a perfect fit for Noctis' plight of being “a rockstar who spurns the spotlight.”
He said the writing team also created a way to show the dynamic freindship of the various characters through something he called "Sorkinization," or dialogue that could have been written by movie screenwriter Aaron Sorkin. A complex diagram of how the characters relate to each other was created and a verbal and nonverbal cues were created using various language techniques that helped the dialogue seem natural.
The team obviously did it right, as our Jason Faulkner said in his Final Fantasy 15 review: "While you’re playing it, you’re an inhabitant of its world, and it’s a place that can be almost as vivid as reality at times, with its own history, lore, and culture. As a Final Fantasy, FFXV returns the series to the wonderment that Final Fantasy VII inspired in a generation of gamers."