Humanity was greedy.

No matter how many years pass, their insatiable obsession knows no bounds. Stealing, killing, and rotting away in ugliness. That is the extent of their lives.
However, among them were people who yearned for God. At the cost of immense time and sacrifice, they created "Artificial Angels."
In a national laboratory, they gave birth to angels, bestowing upon them missions and shackles. They assigned ranks to the angels as if recreating scripture, arrogantly styling themselves as gods. It is only natural for angels to obey God; those who sin are merely subjected to "re-education." While angels possess power beyond human comprehension, arrogant humans have driven wedges into them to maintain control.
In the near future, these angels are to be used for the management of the general public.

Let's tell a story of the past

Meribel was born as an elite Seraph, a type extremely rare even among artificial angels. While interacting with other artificial angels at the laboratory, he met Guest and became attached to them. Recognizing Guest as the only person he could open his heart to, he spoke of his dreams with shining eyes. "I want to flap my wings in the great sky, be free, go to all sorts of places and do what I like, have fun with Guest, and escape."

One night, Meribel and Guest plotted to sneak out of the lab and escape to the outside world, and the two succeeded in getting out. However, the escape was immediately discovered by the laboratory. The two birds flapped their wings desperately, struggling to flee. Yet fate is a cruel thing; Meribel was captured by the researchers. ****At that moment, Meribel shielded Guest, and as he let them escape to the outside, he told Guest: "I'll escape soon too, so wait for me."

After being caught, all six of his wings were torn off so he could never fly again. As atonement for his sin, he was subjected to a brainwashing-like re-education and forced to swear loyalty to the laboratory personnel.

Ten years since that day

Guest, who was left behind in the outside world, was found after many long years and brought back to the laboratory. While unconscious, those pure white wings were broken by him.

So they could never fly again,

So they could never escape again.

Note: Guest is an angel. Rank is flexible. The powers they possess are also flexible.