Clay
A 20-something who loses his job as a marketing and internet guy for a start-up restaurant during the U.S. recession in the 2000's. He finds a day job at a bookstore instead, where he still offers his services as an online marketer. His marketing efforts bring very few customers, but they do attract Kat, Clay's would-be girlfriend.
Penumbra
This mysterious bookstore own also serves as a 24-hour hub to his secret society whose members come in and borrow books without asking, but only from one wall—the back wall, which Clay learns, is full of codes and cyphers. Penumbra tells him of the society—the Unbroken Spine—and their attempts to gain immortal life by decoding this book.
Kat
This romantic interest is a pretty central character. Recruited to the store by one of Clay's online marketing strategies, Kat quickly becomes something of a girlfriend to Clay, and when he goes on his adventure to New York, Kat and another random friend named Neel come along.
Mat
A friend of Clay's who helps him discover the mysteries about the bookstore, finding patterns, clues, and codes around the bookstore.
Neel
A friend of Clay's who accompanies him to New York to follow Penumbra to a meeting of the Unbroken Spine.
Corvina
The archaic-minded leader of the Unbroken Spine secret society which has been dedicated to the deciphering of an old manuscript by a man named Manutius. The society exists to discover the book's secrets for attaining immortal life, but Corvina won't use computers for some weird ethical reason.
Deckle
This Unbroken Spine member helps Clay to find Penumbra after one of his many disappearances, but this allows Clay to help Deckle in a quest of his own: Recovering the missing font plates for the Gerritszoon font.
