Based on the erotic game by Lilith Mist.

Are you still... a human being?

The girl who adores cosplay appeared before you, but you did not know who she actually was. She was staring mutely at me. Unfriendly at all, though she was wearing funny clothes.
My life was completely changed by the encounter with the beautiful girl Sion who once attempted to kill me...

(Source: DLsite English)