Kestrel is a mysterious young man of little fortune, whose parents are dead and who posseses no talent except for music. And, it turns out, he has a talent for detecting. He's an outsider detective, who begins with no real friend except Dipper, and ends having gained a father-figure in McGregor. Just the sort of story I like.
I noticed on this read, however, that the tenets of modern mystery writing had overwritten some of the historical elements; that is, Kestrel's methodology is very much that of the modern police detective and not what one might expect from someone truly of the period. This didn't bother me too much, but it might bother other readers.