The order would go like this: In City of Angels, Seth starts out as an angel, invisible to mortals and on God's errand. But during the course of that film, he falls in love with Meg Ryan, who tragically dies. After coming to terms with this tragedy (or not... who knows?) during that film's Gainax Ending, he moves on, settles down again and starts a new life, changing his name to Milton Red Miller.
This leads to Mandy (2018). His idyllic life with the eponymous Mandy shattered by the events of this film, he is forever cracked and broken... nothing like the angel he once was. He lives a broken, tumultuous live, turns to crime and sleeping around, destining himself to Hell.
Thus follows Drive Angry. When his daughter (that he had either by remarrying, or out of wedlock) and son-in-law are killed by a cult and his granddaughter kidnapped, Milton cannot sit idly by and let Hell get away with it. So he steals the Godkiller and drives out of the infernal realm to set things right. Unfortunately, though he does, he also is doomed to still return to his damnation. But it's obvious he will rebel. He's broken out once before, he'll do it again.
So now comes the Ghost Rider Duology. Having been so rambunctious and having caused so many problems, Milton is turned into The Spirit of Justice, allowing the Devil to wield him like a weapon. But when the devil curses Johnny Blaze with the Spirit of Vengeance, the angel Seth/Milton turns the tables and becomes a crusader for justice. The fact that Johnny Blaze starts out as a young man who looks nothing like Nicolas Cage who grows up to look like him is a sign of Translation Convention: the grown up Johnny Blaze probably still looks nothing like our favorite batshit actor, but having us see the angel's visage instead of the mortal he's attached to is a sign that he's the same character.
So when Johnny Blaze recalls that "there was an angel" in Ghost Rider: Spirit of Vengeance, he's hazily recalling Seth/Milton/Red's backstory.
The man is responsible for the creation of the Black Skulls through just a "taste" of his LSD (possibly a case of Go Mad from the Revelation), Jeramiah was a musician before "hearing the voice of god telling him to leave that life behind" and he obviously knows the Chemist personally (a case of gods words being intentionally misinterpreted into something sinister); after tasting the LSD, being possibly symbolic of tasting forbidden knowledge, Red knows exactly where to find the Chemist.
The Chemist takes his own LSD very casually (having none of the side effects that other people had) and seemingly knows exactly what Red is feeling, what he's thinking and what he wants just by looking at him, you cant even say it was body language, Red is perfectly still the entire time. Upon seeing that Jeramiah has irreversibly hurt Red he quite literally releases the beast from its cage, in this case the beast was a tiger, passing by and being released from Nicolas Cage (that's right a meta pun!) and symbolically giving him permission (and possibly the strength) to enact revenge on the cult.
He is an All-Powerful Bystander who is only seen when he wants to be seen, can create human angels and demons (Red being the former, Black Skulls being the latter), is seemingly everywhere at once (being mentioned constantly), is able to hear your thoughts, know what you're feeling at any given moment and seems to genuinely care about you (getting visibly distressed when Red is hurt).
