3.5 Stars. Upon finishing this book I realized that I should have learned more about it before reading -- and then read the first book and proceeded to read the trilogy in order.
As a standalone, the hero's purpose ends on a seriously lame note as the mystery isn't resolved and the story sort of just...ends (and I would've known this had I better prepared...) and I couldn't help but feel its' lack as a point against it without the context of the first book (especially since the setup was well done.)
Other books where a thread weaves throughout have done this in a far more satisfying fashion for the 2nd entry [go read Tessa Dare's unfortunately titled yet incredible Stud Club trilogy!]
Luckily the characters are interesting and their relationship so satisfying that it still earned 3.5 Stars in my book (imagine if the mystery/ending had been handled better!)
Gerard is one of those loveable, jovial, friend to everyone kinds of guys that can be a breath of fresh air after brooding/wounded heroes.
Katherine/Kate is a heroine I felt for when it became so clear

Spoiler

her mother was basically the worst and had been holding her back and keeping her down (and downtrodden) for years,

which made her transformation that much more enjoyable. I did feel for her, even at her silliest.