Darkness of the Sea, Shadow of the Moon

Ruka and Rumi Kobayakawa, identical twin sisters, are in a complicated relationship with Katsuyuki Touma, an upperclassman with a strong athletic background. Rumi initially appears to support Ruka's budding romance with Katsuyuki, but their lives take a dark turn when they're infected with a mysterious bacterium. This infection drastically alters Rumi's personality, and it also grants the twins supernatural powers with a terrifying edge – powers that Rumi now uses to pursue her obsessive love for Katsuyuki at any cost, even if it means harming her own sister.