What happens between A and Rhiannon in Every Day?

Simply stated, A and Rhiannon fall in love. Their relationship, however, is much more complicated than that.
A is a unique character who wakes up each day in a different person's body. In the beginning of the story, A wakes up in the body of Rhiannon's boyfriend, which is how they meet. They share a special day, and A just can't stop thinking about Rhiannon.
Throughout the novel, A connects with Rhiannon in the bodies of various people and eventually tells her about A's unique condition. The two struggle with a tumultuous relationship as A wakes up in different places and different bodies, some more attractive to Rhiannon than others.
By the end of the novel, A must make a moral choice to "steal" someone else's life or lose Rhiannon forever.