One of the most important themes in Every Day by David Levithan is identity. A is unable to fully create their own identity because they don't have a body or a history. When A falls in love and creates a real relationship for the first time, they become increasingly aware of how their lack of concrete identity hampers their existence.
A lives each day in a different person's body. Through trial and error, they've learned to make as few changes as possible to the person's life. A picks up context clues and tries to get through each day without alerting people that they've taken control of that person's body during the day. When A meets Rhiannon, the girlfriend of Justin, who A is controlling, A falls in love. When Nathan, a boy A controls, remembers pieces of the experience and calls A a demon, A struggles with the idea that they might be evil.
A major shift in A's personal identity occurs when they realize there are others who control people's bodies -- and that Poole, the other entity...
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