There are a lot of characters involved in this story, and I was able to keep them all straight, which means that Oh did a great job at constructing individual personalities for each one, including the spirits. I love Harper's irritated sister and parents, her adoring brother, her supportive friend Dayo, and even her "imaginary" friend Rose, whom she loses. Even Olivia and her troubled family are described just enough that their story made sense and added a lot to the sage of the demons. This sort of reliable character building across the entire population of the book is no easy task!
The draw of this book is, of course, the monsters. They are suitably evil. They glow from the ingested spirits that Harper must release, and are relentless in their pursuit of energy. Harper's growing skills at dealing with them will be interesting to follow in subsequent books, and I love the fact that not all of the spirits she meets are evil-- she runs across a whole range, some of whom are lovely.
Spirit Hunters has all of the best qualities of traditional ghost stories but has a fresh, updated quality with the details of Harper's Korean background and her supportive if somewhat clueless family network.