 If you liked
Ayashi no Ceres
|
 ...then you might like
Yurara no Tsuki
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Ayashi no Ceres and Yurara are somewhat similiar manga. In Ceres, a teenage girl, Aya, can transform into her legendary ancestor, Ceres. In Yurara, high school student Yurara, can call on her guardian spirit for protection. What Yurara does that, her body transforms to look like her guardian spirit. Her guardian spirit's personality also affects her and influences her choices. Ayashi no Ceres is decidedly darker than Yurara, which tends to be more light hearted. Both Ceres and Yurara also feature love triangles between the main female character and the two male protagonists.
In both manga the main girl turns into another woman -- in Yurara it's her guardian spirit, and in Ceres it's her ancestor, but in Yurara the girl has the memories of when she transforms, and in Ceres she doesn't. Both girls have a phase where they absolutely don't want to transform under any circumstances, but sometimes have to because they're transformation can protect them. They both have complicated love triangles.