Speaking to Shack News, (via GamesRadar) the veteran developer is asked how the Final Fantasy 7 Rebirth team approached Sephiroth's design regarding story and gameplay and how you present him as a hero of the time.
Nomura starts by saying that Sephiroth is a more prominent presence this time; "much more so than the original title." While Nomura also remarks that he's a character who "really moves the story along in a very immense and impactful way," he stops short there, saying you'll need to play Rebirth to see what that's all about.
With that cleared out of the way, Nomura ponders what makes the tall, long-haired bad boy so desirable to fans.
As far as Sephiroth being this extremely popular character, frankly, I'm very curious as to what makes him so attractive to users, because I hear this opinion very often," he says. "His appearance, firstly, I'm sure, but also perhaps there is something about his background that brings about a particular appeal to so many of the players that makes him this iconic character. And so, for Rebirth, I would think that besides Cloud, Sephiroth is also a protagonist that we can think of for this title.
Truly, a mystery.