Alexiel is also known as the Organic Angel, born from Adam Kadamon (or Seraphita), along with her younger twin, Rosiel. Alexiel was in love with God, but was jealous of Rosiel since God loved him. Alexiel started a revolt against God and Heaven, fighting her brother as well because he was slowly beginning to lose his sanity. Rosiel made her promise to kill him after he "changed" because he didn't want to be seen as "not beautiful." But when Alexiel was about to kill him, she couldn't force herself to do it, so she sealed him inside Earth.
With no energy to defeat God, Alexiel was captured and judgement was passed that Alexiel's soul would be forever reincarnated inside a mortal body. Right now, her soul is inside Setsuna.