Deuteragonist is the fancy word for a sidekick or a foil for the main character of a story. It comes from classical drama where there used to be one main actor who spoke lines with the chorus. Then a second actor (the deuteragonist) was added to spice things up. The deuteragonist can either complement the main character or serve as a means to highlight opposite character traits of the main character.
In the 2005 film Dark Water the character of Ceci, a little girl who moves into a kind of gothic old apartment building, develops an imaginary friend, Natasha. It’s a common troupe in literature…the imaginary friend who always gets the blame for all the bad things the real child does. But Natasha isn’t a normal imaginary friend…she’s a ghost.
Ceci is a normal child by all standards, but once Natasha enters her world, life for both Ceci and her mother changes drastically. It literally becomes Natasha’s story. The plot twist at the end is interesting and a statement on what some mothers are willing to sacrifice for their children while others could care less. Natasha is very real and a deuteragonist in every sense of the word, And this remake of the Japanese film of the same name is highly underrated.