Isaac is a boy from Hazel’s childhood cancer support group who loses his eyesight as a result of his illness. He and Hazel bond over the monotonous rituals of the Support Group, and his decision to bring Augustus along to a meeting sparks the novel’s plot development. While his cancer journey is not necessarily central to the narrative, it does offer yet another version of what suffering can look like and foreshadows the realizations that Hazel will come to at the end of the novel. The despair Isaac feels is primarily from his girlfriend Monica’s decision to break up with him due to his blindness rather than the loss of his remaining eye itself. He champions the importance of love from the beginning, and as Hazel feels the pain of losing love in the wake of Augustus’s death, Isaac acts as a key source of support for her. Although Isaac may not be able to physically see, he has a unique ability to see beyond the immediate consequences of childhood cancer and find meaning in the world around him.

As Isaac endures losing both his vision and his girlfriend, he expresses a rather cynical attitude toward his condition that highlights the unique struggle of navigating both severe illness and the perils of growing up. He exchanges hopeless sighs with Hazel and makes jokes about his impending blindness with Augustus, both of which serve as ways of coping with the unfair realities of his cancer. Monica’s decision to leave him, however, compounds this sense of helplessness and leads to what Augustus and Hazel refer to as the Night of the Broken Trophies in Chapter 4. Isaac finally lets out his anger by breaking Augustus’s basketball trophies, an act which allows him to feel a sense of power that he struggles to find in his daily life. 

Despite the frustration he feels, Isaac serves as a champion for both love and friendship throughout the novel. He insists in Chapter 5 that “everybody should have true love” and suggests that relationships are the key to pushing through difficult situations, arguing prior to their breakup that Monica’s presence comforts him. Once they do split, however, Isaac continues to find meaning in his life through his friendships with both Hazel and Augustus. All three of them support each other throughout their various health battles by being physically present for one another, refusing to shy away from the harsh realities of their suffering. Isaac attends both Augustus’s pre-funeral and actual funeral, for example, and he struggles to deliver his eulogy because of just how meaningful their friendship is to him. In these moments, Isaac embodies the novel’s primary argument that love and friendship are ultimately worth the pain of loss.