Angel Sanctuary (Tenshi Kinryōku) is a manga that was created by Kaori Yuki. Yuki ended up becoming a mangaka after she continually entered, and won, contests that were given by Hana to Yume. It was in 1987 that she became a full-time manga artist, and in 1995, Angel Sanctuary was first published by the very same magazine that held the contests she won: Hana to Yume. Angel Sanctuary was turned into a three-part original video animation (OVA) in 2000.
Setsuna Mudo is not your normal, average, every day high school student. Instead, he is a student who is constantly in trouble and is in love with his sister. He is also a reincarnation of an angel who fell from heaven, which puts him in the position of being caught in the war between heaven and hell. Because he is the reincarnation of the most powerful angel ever created, he and his sister, Sara, who is a reincarnation of the angel of water, Jibril, find themselves being attacked by other angels, who are scared that Setsuna will be able to use the powers gifted to him to wipe out them all.
A long time ago, there was an angel known as the Organic Angel Alexiel. She was third in command behind God and a man known as Adam Kadamon. She and her twin brother, Inorganic Angel Rosiel, help God to rule, especially when God takes a rest in order to gather his powers for the fight against hell. When God is sleeping, the angels decide to take out the race of beings known as the Evil race. They were cast out of hell. The angels decide to destroy them, and become criminals themselves, murdering in the name of God.
Alexiel has very strong morals, and has a difficult time understanding why the angels should fight the way they did. She ends up taking the Evil’s side of the war, because she believes that the Evil’s are not putting up a front and using God as an excuse for their actions. In doing this, she officially revolts against God.
During the war, Alexiel ends up fighting with Rosiel, her brother. Rosiel had recently discovered that his body and mind were disintegrating, and he asks Alexiel to kill him before he went insane. She sadly agrees, and is about to kill him when he screams at her and talks her out of it. Instead of killing him, she seals him away within the Earth, using magic that is forbidden to her. The angel of judgment, Uriel, takes her to task for this, and decides that she must be punished. He decrees that her body and soul will be separated for all time. Her body will be frozen in an angel crystal, and her soul will be reincarnated into the body of a human mortal. It is also a fact that the mortal she is reincarnated into is bound to die a painful death. It is into the body of Setsuna Mudo that Alexiel is reincarnated.