This is essentially how I'm going to explain Angels and Demons in my story about em. I think this is the best way to say it.
To begin with, they aren't angels or demons in the traditional sense. Demons have lived on Earth for as long as people have. They don't come from an otherworldy plain, they're just another creature on the Earth. They have lived among people, in their cities, doing their jobs, and they are completely inconspicuous. The only thing separating them from a regular person is a very specific tattoo that shows up somewhere on their body. They are very hostile, but not just because they want to do something bad. Whenever they experience some form of chaos, like a death, inflicting injury, creating panic, endorphines get released in their brain. It's like chaos is a drug and they are all addicts. So essentially they continued doing this same routine of terrorizing people and doing it from the shadows in such a way no one could know until this one demon was born. Nobody knows her name, nobody knows what she looked. Only two things abut her were known. 1) she had brilliant green eyes and 2) she was very different. You see demons have this sort of pool of abilities they can have, but each individual one only has access to 1-3 of them. She had an extremely rare gene issue that resulted in her having access to all of them, something only even heard of once in all of demon history, but nobody knows what happened to those two. So the demon girl was watching all of this destruction and death and chaos take place on a regular basis, but instead of getting high off of it, she was APPALLED at it. She decided to fight them all over it, but naturally she knew she couldn't do it all by herself. So she used an ability to create copies of herself, gave them all their own genetics, but they all possessed her green eyes and all abilities. Eventually people started calling the evil demons "demons" and the heroic demons "angels" and each side, seeing as they had no name, decided to take up the moniker demons and angels, and in an attempt to make people feel slightly more secure around them, angels decided to take on the role fully and dress the part when out in public. And thus, the Demons and Angels were created.
Even though angels are themselves stronger than literally any demons, there's far more demons than angels, leading to a losing battle on the angels side.
Angels and Demons both organize their groups in different ways. Angels organize themselves like a police force, with a chief, lieutenants, officers and several other ranks. Demons, on the other hand, are largely an unorganized militant group. Their "king" is more of just the strongest warrior there and he runs the group through fear of crossing him. He has an assortment of generals as he calls them that are simply the elites and one such general owns the Seven Sirens, a small group with the most lethality of all other demons.
Lastly even though they do not usually have names, it is fairly common for both demons and angels to name themselves.