Born from Adam Kadmon to love God, Alexiel rebels against him because of the love she has for Rosiel. She is forced to conceal her feelings under the threat that Rosiel would be killed the moment God heard any declarations of love from Alexiel to her brother. She is also sentenced to be confined in Briah, the Angel's Garden (Eden), and forced to eat the fruit of the Tree of Life. This act is damning her, because the tree is grown from part of Adam Kadmon's body - she is technically eating the flesh of her parent. It is also in Eden where Alexiel and Lucifer first meet. After God had revealed his fate to Lucifer, that he would be the purest form of evil in the world to make God himself appear greater, Lucifer wanted to take revenge by corrupting and killing God's "beloved" daughter. After speaking with her, he changes his mind and promises to break her out of Eden, which he later does. This, however leads to the erasing of his memories and imprisonment of his soul into Nanatsusaya, which Alexiel later finds and fights with.
When she seals Rosiel away, she is imprisoned and tried by the High Council for her actions. After lying that the one she loved most was God, Uriel sentences her body to be imprisoned in the angel crystal while her soul is endlessly reincarnated into a human without end; each of her lives are to be miserable and end in a painful death. Setsuna is the current incaration of Alexiel, and has awakend as the savior, so he has access to all of Alexiel's powers. Throughout the series, there are a few times when Alexiel reawakens. When Setsuna falls into the river Lethe in hell, and at the end when Rosiel awakens as the angel of the apocalypse. The first time, she reveals Lucifer's identity to him, and the second time, she confesses her love to Rosiel, kills him, and absorbs his decaying body into her womb. There, her organic powers will constantly regenerate him, forever. She is the first of the creator's attempts to split the powers of the Seraphita.