The experience of rivalry and the search for identity in the dynamic between siblings is one of the main themes explored in the book. The dual narrative itself emphasizes the element of competition that exists in the story, as well as heightens the need for a search for unique identity.

Noah and Jude are twins; besides a birthday, the twins share their natural, artistic talent. With respect to the relationship they share, the earlier timeline displays how for a significant portion of their lives, Noah perceived the two of them as sharing one soul; when he depicted himself and Jude separately in his art, he would show them as having only half a body. Despite the rift between them, Jude, too, acknowledges the dynamic that existed between them; the first sculpture she works on is a conjoined Noah and Jude. The bond between Noah and Jude as twins, beyond just being siblings, is an intense one. This is highlighted in the depth of Jude’s anger and heartbreak in the few moments that Noah stops breathing when he jumps off Devil’s Drop and almost drowns.

Despite these significant similarities, however, the differences between Noah and Jude are obvious and numerous from the outset.