This is canon to the series of Непета Стражлке.

Baku is an ancient spirit who possessed people for fun and used them.He has a book called The Book Of Dreams and he protects it because the book is his lifeline.If the book is destroyed,Baku dies.The book contains ancient riddles and incantations for summoning his spirit and other spirits.If the book gets into the wrong person's or an evil spirit's hands,them Baku must obey them and will be under their control.

Sonya Mayer is a 16 year old girl who often gets spirits to attract to her.

Morpheus is an ancient spirit who often helps Sonya with the paranormal.Morpheus was originally found by Sonya's great great grandmother with the same name.After seeing the resemblance between Sonya and her grandmother,Morpheus didn't ever leave the mirror,helping Sonya whenever she needed help.

After an accident when Sonya and her identical twin cousin Nastya,tried summoning a spirit, Baku possessed Nastya and fooled around with Sonya,refusing to leave Nastya's body.

After Baku left the house,still in Nastya's body,Sonya quickly got a mirror called Morpheus for help.Morpheus,told Sonya to go to the nearest churchy because there was hidden in a graveyard Baku's book,and that she should take it out of the ground and open a specific page to read the incantation of forcing Baku of the body he possessed and making Baku get stuck in the book.

Skip to after all this.

Baku only could speak from the book if he wanted to annoy Sonya or Nastya,but he couldn't get out without Sonya reading the incantation of summoning him outloud.

Sonya kept the book in her room and didn't free Baku yet,despite how annoying Baku was.

(You're Sonya.)