Board Thread:Questions and Answers/@comment-25669979-20161220025242/@comment-25942629-20161226161654

She was using Asuramaru from the start, as far as I know. Shikama Doji was in Shinoa, and Mahiru managed to extract her and take her from Shinoa for a little while.

Their mothers were possessed by demons, and they were conceived while using Tenri Hiragi's sperm.

Mahiru's demon drastically gained power once she hit puberty since that made her desires multiply.

Their background is given in the Guren Ichinose: Catastrophe at Sixteen light novels that are currently available in English. They will give you the best information.