This is gonna be sooo messy because I have a headache and I'm queasy as hell, I'm so sorry.
My theories in short: MC was Uriel the archangel, who fell, and Sister was born from MC's essence/soul being split in half.
Theory in long:
If we are to assume the Michael, Gabriel, and Raphael in the prologue are the actual Archangels as in religion, I'm gonna throw out the suggestion of MC being Uriel, as the fourth most known archangel, who, according to Wikipedia:
Uriel is often identified as a cherub and the angel of repentance. He "stands at the Gate of Eden with a fiery sword", or as the angel "who is over the world and over Tartarus."
In the Apocalypse of Peter, he appears as the angel of repentance, who is graphically represented as being as pitiless as any demon.
In modern angelology, Uriel is identified variously as a seraph, cherub, regent of the sun, flame of God, angel of the divine presence, presider over Tartarus (hell), archangel of salvation, and, in later scriptures, identified with Phanuel ("God has turned").
That can explain the reactions of the trio at the start, swinging between asking MC to repent, and looking at them in disgust/betrayal. It feels personal— if MC was just some mortal that got powerful through Lucifer, they wouldn't have hung around long enough for Lucifer to come and save MC's tail.
AND Uriel's unique position at being so close to Hell can justify how they fell: "He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." and whatnot. More to this part is:
"Your claims mean nothing," Michael replies with cold certainty that resonates through the air. "Your history—your nature—makes you the prime suspect."
Which, again, Uriel's position would certainly make Michael say something like that if what had happened at the beginning involved Hell and demons (which it clearly did with Lucifer's presence).
Also, "By force of habit, you shrug your shoulders, but the movement only reminds you of the weight that is no longer there." which suggests MC used to have wings, so at the very least they were part of the angelic hierarchy.
Hatred isn't something new to you, but hatred from Raphael is. It lingers within you even as the archangel turns away.
This part, if nothing else, suggests whatever MC was in the past, they were close to the trio enough that Raphael's gaze at MC was at least friendly, if not affectionate. Michael as well asked MC to come back and repent when they still could— he was ready to accept MC back.
As for MC's sister, I don't know how to explain it but I think she was created out of MC by the "Creator". When, I'm not sure.
I was initially tempted to say "when MC fell" since the Creator "reached" for them during that, but I now wonder if it happened before the fall and was part of the reason why MC had fallen to start with. Perhaps a misunderstanding behind the creation of Sister (the Creator wanted a companion for MC, MC thought they were being replaced)?
Looking into the eyes of a new life—a new you, but better, more magnificent, you lean in to ask, 'How does it feel? Being loved by the Creator?' Your tone is full of hatred. In response, you see a spark in the gaze of the creation He's molded out of you. Her eyes are filled with love—evidence of your connection.
"How can I not?" She turns to you, her expression no longer warm and familiar. There's a haunting emptiness in her [same color as MC's] eyes, a distant gaze that sends a chill down your spine. "Didn't you notice?"
She speaks your name, both familiar and foreign to you, and though it echoes with a hollow sound, causing fear to grip your chest, your heart swells joyfully with recognition.
(So many different ways for me to interpret this section... MC subconsciously recognizing the other half of their soul? MC feeling happy to hear Sister call them by their name, cementing them both as different beings instead of Sister being a "better MC" as in previous past section?)
While I won't confirm or deny anything, I was really happy to read certain parts of your analysis!
Thank you for sharing your theory!














