Alina
Alina is a young girl from Ravka who has the power of harnessing light, which makes her a rare Sun Summoner. Symbolically representing morality and goodness, Alina channels her strength and courage in order to defeat The Darkling.
The Firebird
The Firebird is an amplifier, who is able to fortify powers. Alina is in great need of this, and therefore wants to use The Firebird's powers in order to restore her strength, needed to fight The Darkling. In the end, it is discovered that Mal is actually the amplifier.
Mal
Mal is Alina's close friend, who we meet earlier in the series. Mal is a loyal, dedicated person, who helps Alina to defeat The Darkling.
The Darkling
The Darkling is the antagonist of this novel, who is set on encasing the world in shadow. The Darkling is motivated by his desire for Alina and tells her that he won't give in until she desires him back.