Hannah Baker, the protagonist of Thirteen Reasons Why, narrates the story with stark and bitter honesty. Saddled with a reputation that has little basis in truth, she is essentially friendless and isolated, and longs for people to look past the rumors that plague her and to see her as she really is. Hannah has been victimized by gossip and innuendos that objectify her and imply that she has a promiscuous nature. The few students with whom she tries to connect as friends invariably end up betraying her; as a consequence, Hannah finds that she is always on guard.

Hannah endures relentless cruelty from her peers. Justin, with whom she shares her first kiss, spreads rumors about her; Alex puts her on a list that leads others to view her as a sexual object; and Tyler invades the sanctity of her home. Jessica then slaps her because of provocation from another; Courtney uses her for her own convenience and falsely sullies her reputation still further; Zach steals her encouragement; and Ryan steals her thoughts. Although there is no doubt that Hannah is a victim, there are enigmatic elements in her personality that cause her to contribute paradoxically to her own difficult situation. Hannah does not balk when Courtney invites her to stage a steamy scene to trap Tyler, nor does she initially protest when Marcus begins his sexual advances at the diner. When she finally connects with Clay, who offers her the caring relationship she craves, she panics and rejects him without explanation.

Hannah's character is complex and tragic. She is human and eminently fallible; knowing better than anyone the damage that can be caused by gossip, she cannot help but relate information about Tyler that would better remain unsaid. As he listens to her story, Clay gently chides Hannah for perversely acting like those who persecute her when she talks about Tyler; after her death, her words result in Marcus and Alex feeling free to terrorize Tyler, throwing rocks at his window because they think he is "a freak." It is clear that Hannah is driven to the point of self-destruction by the cruelty of others, but in the end, the awareness that she shares the darker aspects of human nature along with her tormentors, coupled with the knowledge of her complicity in her own victimization, only adds to her self-loathing, and hastens her to the depths of despair.

Clay Jensen has a reputation that is the polar opposite of Hannah's. He is the perennial "nice guy," and no one has anything bad to say about him. Clay does, in fact, have a basic wholesomeness and decency that seem to define him, but his faultless image also stems partly from the fact that, before Hannah's death, he is largely passive. Retiring by nature, Clay does nothing bad because he tends not to do very much at all, preferring to be an observer rather than an actor in the drama that surrounds him. Clay is the one character who really grows in the narrative. As he listens to Hannah's tapes, he remembers his own reactions to some of the events she recounts. Clay had known that it was Justin who gave Hannah her first kiss; he had seen Hannah's name on the "Best Ass" list as it was passed around the class and had initially thought that it was funny. Clay, in effect, interacts with Hannah through his thoughts as he listens to the tapes, lamenting his own obliviousness most of the time, but also discerning places where he believes Hannah is being illogical or unfair. Clay is intelligent and introspective, and because he really thinks about what she is saying, he is able to learn from her experience and grow to become a better person. Instead of being overcome by the horror of the story he hears and crushed by the realization of the role he played in it, Clay shoulders the responsibility for his shortcomings, taking concrete steps to prevent making the same mistakes again by reaching out to another who might be in the same position as Hannah was, Skye Miller.

The other characters who are accused of contributing to Hannah's demise vary in the degree of their malevolence. For the most part, they are thoughtless, with no idea of the ramifications of their actions. Justin, who first kissed Hannah and later stood by while Bryce raped Jessica, is weak in the face of peer pressure, while Zach, whom Clay says would never have done the things he did if he had known how terrible they were for Hannah, is petty but essentially decent, refusing to badmouth her to his friends even when they ridicule him after she does not respond to his effort to get a date. Others, like Marcus and Alex, who terrorize Tyler in his own house after they hear about the "Peeping Tom" incident, are more malicious in nature. Bryce, whose treatment of women is beyond abominable and who rapes an unconscious girl before taking advantage of an obviously distressed Hannah in the hot tub, is positively depraved. No single character sets out to destroy Hannah, and taken individually, most of their actions were stupid and hurtful, but not in themselves grievous. The tragedy of the story is that because each person's role in Hannah's destruction was comparatively small, most of them would have never understood that the accumulation of their actions created an environment for her in which living became untenable.