Who is the villain?

(Please ignore the book cover, it was the first thing I saw in my camera roll 😭😭😭)

In a world of chaos and destruction, three detectives, Mika, Lilie, and Boxie are after a mysterious criminal. Its crimes start out small, but start getting more serious over time. When they almost have it in their grasp, it always manages to slip out and escape easily. It always left codes to break and messages, taunting them, but giving them hints at the same time. They are slowly being driven to the brink of insanity, and all of them are starting the same: what if they turned to the other side? What do you think? Will they become villains? Or will they hold on the their sanity and stay on the right side?

(you can contact me through Pinterest if you want to make suggestions for the book or you have critism!
My Pinterest: @theofficialdazai)