I hate Whitney. I really don't care about Doug. The first sigh of a problem. But the further you go, the wore it gets.
Whitney is an entitled, spoiled and... an idiot. Examples:
When Dough saves their life by a dramatic jump from a speeding train she does not say "thank you". No. She yells at him for the bruises she got from it. She then proceeds to get into a tantrum. One of may, I might add.
Every time he tells her to run and/or hide she responds basically by "you will not tell what to do!" routine.
When he brings her a disguise so she could blend in (and not get spotted and killed) she refuses and again bitches about the ugly clothes and that she will not be wearing them. She basically wants to be fashionably dead.
Also - they are trekking through jungles of Madagascar so of course she chose to wear A SKIRT. And an elegant blouse. Because of reasons.
She does not learn anything, she does not adapt to a situation. Her survival instinct is nonexistent.
Doug is, well just bland. But I have no idea what he sees in Whitney. And he is really well.... let's just say that - it is not enough to write that your character is cunning smart. You have to show it too.
There is also a treasure everyone is after. And Doug does not want to tell Whitney what the treasure is. And because of that, you as a reader do not get to know, too. This is a weird creative choice, to be honest. As readers of the story, we don't know what the stakes are. Alas, I was not invested in the outcome at all.
There is also Dimitri, the bad guy. And his goons. Goons are pathetically bad at "gooning". Dimitri is supposed to be bad because Doug says so. But through 90% of the book, Dimitri does not do any single thing that would prove his "badness".
Honestly, at some point I found myself rooting for Dimitri. If he would catch and killed them, the book would end faster. And be more pleasing actually.
"Romancing the stone" this is not.