Too many holes in the plot. Start with a demon named Dante (how cutesy). He is a total badass soul collector. He's so good at it that he's in charge of training other collectors. He's in line for a promotion if he can carry out his latest assignment from Boss Man (i.e. Satan).
He's totally remorseless and enjoys tagging people with seals for their sins, no matter how big or small they are. Then he meets his intended victim, the homely and gawky Charlie Cooper, and he suddenly turns into Dante the Hallmark Demon. He waxes poetic about how wonderful she is and her inner beauty and why he's such a slimeball for corrupting her soul (isn't that his job description?). Eventually he decides he's going to defy the being that runs the 10 levels of Hell by sneaking Charlie away and hiding her until Satan gives up looking for her. Nothing could go wrong with a well thought out plan like that could it?
The flaws in the plot that made me downgrade the book [SPOILER WARNING]:
1) The collector program is too ill-defined. Dante has only been involved for 2 years but he's the senior demon and has trained the 5 others (all of them apparently dead teenage boys). So is this a brand new program Satan has just dreamed up? If so, why later on does it say Dante has heard about a collector that elected to permanently die rather than return to Hell, which would give the impression the program has been around a while. Why is it limited to only 6 demons, who quite frankly don't seem to do a whole lot? They just go around adding black marks to people's souls for bad behavior, which with only 6 demons and 250 million people in the US alone doesn't seem to make much of an impact on the good vs. evil scorecard.
2) Why are all 6 demons apparently dead American teens who seem to focus on the US? I guess Boss Man isn't interested on opportunities for evil in Asia, Europe, Africa, etc.? Nope, just 6 in the US. He shouldn't be running out of space since Dante references the traditional 10 levels of Hell from "The Inferno." So why such a half-assed effort?
3) Dante has the bright idea that he and Charlie are going to hide out among the millions of people living in Tokyo until the collectors get tired of hunting for them. Maybe the demons have the ability to speak whatever language they need to, but for the most part they appear to be clueless teens who need Dante to tell them how to tie their shoes. If not, why can't Boss Man find some locals to look for them? Because how well are a beefcake guy and a beautiful girl with a limp (who doesn't speak Japanese) going to blend in? My daughter spent 3 weeks in China and it was blatantly obvious to all that she wasn't a local. And since Dante bought the plane tickets with the Satanic Black Amex wouldn't that be a clue where they went (he books the tickets then draws out the maximum amount of cash he can)? All Boss Man has to do is check his monthly statement and he'll know where Dante and Charlie are. Not to mention that Charlie is a dirt poor orphan who most likely doesn't have a passport, so how were they going to get past Customs?
4) He never explains how they're going to successfully hide in Tokyo. He's wearing an ankle bracelet that tracks his current location so Boss Man knows where he is. If he removes the bracelet he will permanently die within several hours. So how does he think they're going to escape a very pissed off Satan? Not to mention it's one of the most expensive cities in the world to stay in, so whatever money he managed to swipe won't be enough to live on for long.
5) Apparently the heavenly forces have been doing their own tagging of souls to counteract the collectors but somehow in 2 years of work Dante has never seen a pink seal on anyone. Maybe there are only 6 liberators at work?
6) The total stretch that of all the people in the world, Valerie winds up as a liberator while her ex-fiancé Max is a collector defies credibility. If she's so sweet and he's such a scumbag, how were they ever engaged in the 1st place?
So, sorry but too many gaping holes in the plot to rate this any higher.