BEWARE OF UNMARKED SPOILERS—You Have Been Warned
A Senior undergraduate student at Pollard State University, a (fictional) university in New England. She's majoring in Literature, has a special interest in Library Science and works in the university library. It is there that she finds the Ship of Theseus book, and starts writing in it, thus starting a written communication with Eric (which later leads to them meeting for real).
- Character Blog: Her character has a Twitter account, which offers additional observations on top of those she made in the book, as well as some personal communication with Eric. She also has an account on Tumblr, which houses one of the book's multiple alternate endings, and the only one officially acknowledged by Doug Dorst.
- Conspiracy Theorist: Jen suspects that somebody is constantly following her as she and Eric are getting closer to revealing Straka's identity. The not-so-accidental fire of her parent's barn fuels her fears.
- Do Not Call Me "Paul": Downplayed example because she still uses a derivative of her official birth name, but one of the first things Jen tells Eric (and the reader) when she starts writing in the book is to never call her "Jenny", and only call her "Jen".
- Foil: According to the author, Doug Dorst, Jen was created as a foil to Eric (her character contrasts with his).
- The Watson: According to J. J. Abrams, Jen is designed to be this for readers, as she comes onto the scene knowing little to nothing about Straka and learning about him at the same pace as the readers.
A 20-something guy who previously finished an undergrad education at Pollard State University and would have been a graduate student (in Literature) at the time the book takes place, if not for the fact that he was kicked out of university. Originally from California, he pretty much cut ties with his parents, whom he feels estranged from, when he moved to New England to go to University. He apparently had a falling out with his professor, Moody, and the T.A., Ilsa, which lead to his being expelled. He has had a copy of Ship of Theseus since he was 15, and his leaving this book in the Pollard State University library and subsequently Jen finding it and writing in it, is what sets the plot in motion.
- Author Avatar: According to the author, Doug Dorst, Eric was based on himself.
- Character Blog: His character has a Twitter account, which offers additional observations on top of those he made in the book, as well as some personal communication with Jen.
- Conspiracy Theorist: Eric thinks that his expulsion from the university was orchestrated by his dissertation adviser Professor Moody.
- Dark and Troubled Past: Eric drops hints from the beginning on that he's estranged from his parents. When the full extent of this back story becomes clear, it's even sadder than it seemed at first: he suffered from severe panic attacks, anxiety and depression as a teenager, his parents only scolded him for it instead of seeking treatment and sent him on a prolonged boat trip with his uncle Zeke, during which his symptoms worsened which led Zeke to dump him out of the boat and leave him behind in a town. Zeke afterwards drowned and Eric's parents blamed Eric for this.
- Do Not Call Me "Paul": Eric reveals at a certain point that he has a different birth name / name he grew up with (Nicodemus John), but he chose to call himself Eric once in college. Jen agrees that his original name was an embarrassing one, but that's not Eric's main reason: he mostly chose his new name because he was/is estranged from his parents and wanted to cut ties.
F. X. Caldeira
A Brazilian woman who's the (In Universe) translator of most of V. M. Straka's books. Though Caldeira never met Straka in person, she corresponded with him for decades, and fell in love with him.
- Faking the Dead: She faked her own death in 1964, and has been living since then under her new identity of "Ermelinda Pega".
- Omniglot: She is fluent in many languages, to the point that she is Straka's translator for all translations of his books (which are in over a dozen different languages).
- Samus Is a Girl: Only known as F. X. Caldeira and everybody In Universe assumes without any question that they're a man... except for Jen, who realises Caldeira is in fact a woman, Filomela Xebregas Caldeira.
- Unrequited Love: As Jen and Eric find out as the book progresses, she was vehemently in love with Straka for decades and even used footnotes she put in her translations of his works to communicate this; however, Straka either never was aware of her love, or if he was, ignored it because he either didn't feel the same about her, or he did feel the same but didn't want to get together with her because that would put her in danger.
V. M. Straka
The (In Universe) author of the "innermost" story of the book, which (In Universe) is a book called Ship of Theseus—his 19th and last book. His identity is unknown and subject to much speculation. He (probably) died / was murdered after having finished Ship of Theseus.
- Oblivious to Love: His translator, Caldeira, is in love with him and even uses coded messages in her footnotes to communicate that; he, however, doesn't seem to ever have had a clue about this (though an alternate interpretation is that he did know but pretended not to, either because he didn't reciprocate the feeling, or because he did love her back but thought her being with him would put her in danger).
- Omniglot: He is fluent in many foreign languages; not so good as the translator of his books, F. X. Caldeira, is, but enough to sometimes have a different opinion about a translation by Caldeira, and start to bicker with Caldeira about it.