Ayashi No Ceres or Ceres: Celestial Legend by Yuu Watase
The characters are great, the romance is swoon worthy and the story is fantastic. So, why do I feel a little resentful towards this series? THE ENDING. After 14 volumes of watching Aya struggle with her identity, destiny, place in the world and future with Tooya, I was really invested in her. I loved this girl. The final book didn’t ruin that, there was a happy ending. It just wasn't as happy as I felt the series deserved. It is a dark manga with violence, sex and a little bit of unintended incest. A great series over all, but be prepared for the ending to disappoint a little.