Now that Spider-Man's gone, this city will never be the same.
~ Gustav Fiers.
Gustav Fiers (or The Gentleman) is the main antagonist in Marc Webb's short-lived The Amazing Spider-Man franchise. He is arguably the secondary antagonist of The Amazing Spider-Man and a major antagonist in The Amazing Spider-Man 2.
He appears to be a representative of Oscorp and later associate of Harry Osborn, assisting him in forming the crew "The Sinister Six". Fiers first appeared in a post-credit scene in the first film in which he was credited as The Man in Shadows, but was identified at the end of the second film.
When an Oscorp employee Dr. Curt Connors is locked up at Beloit Psychiatric Asylum, he is approached in his cell by Fiers who asks Connors if he told Peter Parker the truth about his father, in which Connors had replied no. Satisfied, Fiers said he would leave Peter be now, which leads to Connors demanding him to leave Peter alone, in which Fiers simply vanishes.
The Amazing Spider-Man 2
Fiers visits Harry Osborn in his cell at Ravencroft Institute in which they plan to set up a team, and Harry requests to keep it small. Harry then sends Fiers to the Oscorp tower which has weapons and equipment needed for their goals.
Gustav Fiers was previously the main antagonist in the Adam-Troy Castro's trilogy of Spider-Man novels.
At the time of the release of The Amazing Spider-Man, the man's identity was a secret, so he was simply credited as "Man in the Shadows". His identity was later revealed in The Amazing Spider-Man 2, where he was also credited as "Gustav Fiers (The Gentleman)" or, as referred to in the film, Mr. Fiers.
It is rumored that the man who tried to kill Peter's parents on their plane was Gustav Fiers. However, if this is true, then it's unknown how Fiers survived the plane crash, as the Parker couple didn't.
Interestingly, in both movies he appears, his face is never shown, only the silhouette of his body is seen and he visits someone who is incarcerated.
Since The Amazing Spider-Man series was cancelled and Michael Massee passed away in 2016, audiences will never see his face ever nor discover his true motives.
Indeed, given Massee's passing in 2016, it's completely unknown what would have been made with Gustav Fiers in The Amazing Spider-Man 3, which was to be released in 2018. However, it's likely that had the film been filmed, the role would have been recast or written out.
It was presumed that Fiers would be the main antagonist in the Sinister Six film as it was going to be apparently a redemption story. However, all The Amazing Spider-Man spin-offs and sequels were cancelled after Marvel and Sony made a deal to integrate Spider-Man in the Marvel Cinematic Universe.