Rhiannon's relationship with Justin
Rhiannon ironically stays with Justin despite the fact that he treats her poorly. Often, he ignores her and starts arguments with her. Typically, people in relationships with partners who don't like them stay in those relationships; Rhiannon, however, does stay with Justin for much of the novel, showing her low self-esteem.
A
Despite the fact that A changes bodies every day, they know more about themselves than people who stay in the same body every day.
Rhiannon
Rhiannon initially believed that love should be based on consistent, physical attributes. However, she falls for A, whose only constant in their life is inconsistency.
Nathan's posession
After A spends a day in Nathan's body, Nathan believes he is possessed by the devil. Readers know that A isn't a bad person and that his intentions are far from malevolent (unlike the devil). He simply wants to connect with and understand humanity better.