Darkness of the Sea, Shadow of the Moon

The story centers around Ruka and Rumi Kobayakawa, twin sisters, and their complicated feelings for the same man, Katsuyuki Touma, an upperclassman. Initially, Rumi appears supportive of Ruka's budding relationship with Katsuyuki, but their lives take a drastic turn when they contract a mysterious bacterium. This transformation not only alters Rumi's personality but also grants the twins supernatural psychic abilities, forcing Rumi to become increasingly possessive of Katsuyuki, even if it means harming those closest to her, including her own sister.