Myrtu is played by Javier Botet. This is not the first time that Botet plays a horror girl, since then he played Niña Medeiros in the REC (2007)'s franchise as well as the feminine ghost in Mama (2013), and most recently as the Crooked Man in The Conjuring 2 (2016).
Mary Lambert, director of Pet Semetary, appears as Oliver's grandmother in a photo that Piki throws into a plastic garbage bag.
External parts of the house that is shown in the movie is the house where Rudyard Kipling was born. The stuffed tiger toy in the movie is called Khan and the book the mother keeps reading to her son is Jungle Book. Probably the maker's way of paying tribute to Kipling.
Myrtu is named after the Sanskrit word "mrti" which means "death". The god of death in Hinduism is Yama, sometimes identified with Kala, god of time, as time consumes everything and finally brings death. Shiva is a male deity; he presides destruction which allows for new creation by Brahma.
The dot Piki wears in the middle of her forehead is a religious symbol worn by Hindus. As well as being a beauty spot, it also represents the third eye of spiritual sight capable of seeing things the physical eye does not see.