The Boxes is a 1998 young-adult sci-fi/fantasy novel by William Sleator, which draws on the myth of Pandora's Box in modern times.
The protagonist, 15-year-old Anni, is charged by her uncle with keeping a pair of boxes safe. Naturally, she becomes obsessed with opening them, and soon does, releasing a telepathic creature that begins to reproduce. At the same time, a mysterious developer contacts her, offering wealth and success in exchange for her use of the box's power to slow time.
Anni's aunt is abusive, and her uncle is often absent; instead, her friend Henry proves her best ally, helping her without thought of recompense. He is the one to draw comparisons with Pandora's box, and his family is the only one other than Anni's to resist the development company. Anni tells him her secret, and he acts as a soundboard and helper. She describes him as "the nicest boy [she] knew," and although he is interested in her, they use their time figuring out how to stop the developer and contain the creatures, which are spreading and mentally getting inside Anni's head. Henry's most important trait is that he does not demand anything from Anni, who is accustomed to helping others; instead, he helps her, and she matures because of it.