Rei: COMPRENDE!? Mikomi: Yeah. I comprende. Rei': No, no, no, no, no, no, no, no. I comprendo. Yo comprendo. Conjugate the verb, for pity's sake!
Big "NO!": Mikomi, upon learning that Rei really is her sister.
Bittersweet Ending: Mikomi finally succeeds in bringing Rei back to her side, but she quickly loses her to a fatal attack meant for her. However, a few years later, the two sisters are reunited in Heaven, where Rei went after her sister had prayed for her soul, after Mikomi succumbs to a terminal disease.
Due to the Dead: After her sister's death, Mikomi says a prayer for the salvation of her soul and burns her body in a funeral pyre as her comrades watch. She then scatters her ashes at the entrance to the school they attended together as per her last request.
Rei: Faith never told you about your sister. Mikomi: She told me enough! She told me I have no sister! Rei: No, Mikomi. I AM your sister! Mikomi: NOOOOOOOOO!
Moral Event Horizon: Ultimately subverted with Rei after she breaks her own sister's neck and leaves her for dead. Mikomi still has hope for her sister's salvation even after what she did to her, and despite her comrades' initial objections she lets them know that "even though what she did to me was wrong, she's still my sister. And I will win her over even if it costs me my life!" Her hope is confirmed in the last couple of episodes when she first wins her sister over and then successfully prays for her salvation after her death.
Playing Against Type: Emma Watson voices both sisters. While one of them, Mikomi, is on the heroes' side, the other, Rei, fights for the villains for most of the series, thus playing this trope straight.
Psychopomp: Following Mikomi's prayer for Rei's eternal salvation, an angel shows up at the spot where her soul is waiting, and the two ascend to Heaven right before Mikomi sets her Earthly body on fire.
Redemption Equals Death: Rei. She does get to fight on her sister's side once before getting killed by the demons, though.