A short form of angel sinners.
They were once angels who lived peacefully in the heaven. Originally, they were known as warriors in the angel world. They fight the evil and they perish it. They have been trained to fight since they were young.
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Some humans believe angels and demons exist and some don't. However, these beings already exist since the beginning of the world and they exist to test the humans whether they deserve to live in heaven or hell in the afterlife.
Angels are born to protect and guide humans so they must be as pure as snow. But of course, no one can be perfect.