Let's be real: Private Dibdin, one of the prisoners of war Isabel encounters as she seeks to help Curzon, is a serious creep. He has a really bad temper, "His breath [stinks] of rotting teeth" (37.49), and worst of all, he withholds food and provisions from Curzon despite being a fellow soldier. "He's a slave," Dibdin tells Isabel to justify his actions. "He will not be treated same as free men" (37.49). Basically, Dibdin is a good old-fashioned racist. And let's face it: His name isn't all that great either.
So what purpose does a nasty dude like this serve in our story? For one thing, he moves the plot forward by getting Isabel carrying messages again, even though she swore she'd sworn off the espionage thing for good. In exchange for treating Curzon like a human being, she agrees to aid in communication between the prisoners and their Captain.
For another, Dibdin is (yet) another character who reveals the ugliness of slavery to modern readers. It's the middle of winter in the days before prisons had heat and regular meals, and Curzon has no blankets and no food. When Isabel returns, he's dying, and it's largely because of Dibdin's bigotry. He's not a nice guy, but then again, prejudice isn't very nice either.