Daphne is a quiet teenage girl who is half-demon and half-fallen angel. She's the daughter of Lucifer and Lilith and her sisters are seductive soul-sucking succubi (sibilance!). However, Daphne is more like her brother, Obie, whose gentle nature and genuine kindness make him an oddity in Pandemonium. When her brother leaves to make a life for himself on Earth, Daphne finds herself alone, confused and unsure of her future; but when she learns that her brother has been kidnapped by a psychopathic archangel she realises that she will stop at nothing to save him, even if that means going to Earth alone and facing the risk of being hunted and destroyed by Azrael.
Her only clue, as to her brother's whereabouts, comes from a human boy, Truman. However, Truman is a broken young man, and before Daphne can get his help to save her brother, she has to first save Truman from himself. Daphne and Truman make for a strong central pairing, with both characters being really genuine and sympathetic protagonists, with Daphne's innocence and determination, and Truman's intrinsic goodness and tragic past, and their inherent connection making them very easy to root for. The narrative alternates between the first person, from Daphne's point of view, and the third person, following Truman. It's an interesting decision that means that Truman remains more of an enigma, while we really get to know Daphne. The gradual development of Daphne's narrative voice from being distant and observant, to becoming more passionate and emotional, subtly conveys her growth as a character, and forms an important aspect of the book; she is like a blank canvas at the beginning of the book, stuck in a rut in a timeless world and unsure of her purpose and identity, and her journey of self-discovery and finding love is mirrored by that of Truman's, towards redemption from his depression and alcoholism.
Hell's characters, from the suave Beelzebub to the restless Lilith, are generally painted in a better light than the archangels, making for an interesting unconventional portrayal. Pandemonium isn't as hellish as you would expect it to be and Daphne herself is a very sympathetic character; in fact, it would've been intriguing if she'd been conveyed as more morally ambiguous.