Immortality
The main theme of the novel, apparent from the title itself, is the theme of the possibility of immortality. Sebastian is at the center of this exploration of the possible consequences of seeking immortality. In his past with Agnes, he is driven to madness desiring to live forever, and doesn't care what it takes to reach that goal, even stepping over the person who loved him the most, the person whom he oversaw by this hunger for forever. In the present with Evie he realizes these consequences, realizes the loneliness that comes with watching the time go by from the shadows.
Love
The novel portrays the theme of love as something that matters the most in life, something that is worth fighting for even if it seems like one is walking through the dark without light. Evie decides to fight for her impossible love with Sebastian, a love that poses a threat to both their lives. Sebastian is ready to sacrifice himself to save Evie. Love makes both of them selfless.
Friendship and kindness
The novel is set in a school setting and it shows how friendship grows out of mutual kindness, a friendship that makes an individual stronger. Evie longs for a friend, she is alone in a new world. The first person to show her kindness is Sarah, and they soon become very good friends, their friendship becomes even stronger the more hardships arise. Evie has made another friend, that looks out for her unnoticed. Even though Helen seems cold towards Evie, she's been looking out for her from the very beginning, trying to prevent her from getting in trouble. It is this kindness that girls show to each other that makes them even more than friends towards the end, it makes them sisters.
