Shouko Kariuusai lent Yaya to Raishin because she wanted Yaya to drain Raishin's vitality. It is clearly stated in the novels and in the anime. When she met Raishin by the burnt Akabane state, it is even hinted that she would be satisfied by just using his body right there.
Maybe it's due to the inner potential of the Akabane bloodline to have their bodies made into machine dolls (like Nadeshiko).
So, what does she want to do with Raishin, after all?
