To answer your questions about her to my viewing audience of the internet this happen to be Mayuka's water demon form, which you can say it's a bit similar to the beast transformation of the tail beast from Naruto. If you have further questions about her you can ask the owner of this character.:)

Drawn By: RB9

Art Style:Cell Shade/Visual Effects

Her Human Form:http://rb9.deviantart.com/art/RQ-The-Mistress-of-The-Mist-518322148

Mayuka Ginjinnin (The Water Demon) belongs to Oscoulapix

Theme Music:https://www.youtube.com/watch?v=DZ-sDySKwyQ