I'm a librarian who loves anime, manga, and reading a wide variety of genres.
It's kind of fun to play "guess the love interest." (I have no idea if this book is going to have a love interest, but so many of them do.) At the moment, there are three possibilities.
First, Jen's ex-boyfriend. He's a good-looking cop and he's the only guy Jen has had sex with during the course of the story, but he also cheated on Jen with her half-sister, so he's probably out.
Second, Jen's coworker. He's hot, willing to let her cry on his shoulder, and he and Jen have had sex before, but she decided he wasn't really interested in more than that.
Third, there's the private investigator. He and Jen have a past - he humiliated her in high school by announcing in public that he'd slept with the school's last three virgins, one of whom was her. Now, years later, he has apologized for this and is attempting to help her with recent events (a murder, a burglary, etc.). He hasn't really been around much yet.
Jen's coworker seems like the most likely option. He seems too nice for her though, so I kind of hope that he ends up with one of the cafe's part-timers.