Colonel Dick is a Union officer, which is interesting because he's one of the most decent human beings in this portrait of a Confederate family. We're made to root for the Southerners in many ways because Bayard and Granny are pretty likable folks (if you can get past the way they treat their slaves, Ringo excluded).
So Col. Dick, as a Northerner who is a good guy, sort of screws up the black-and-white vision that most of the characters seem to have. Which goes something like this: Yankees = Bad, Southerners = Good.
He saves the boys from his own troops even after they shoot one of his horses, and he also makes sure Granny gets back her confiscated property, and then some. Col. Dick doesn't really have that big of a role in the novel, but his intervention is always good, making us question the other characters' tunnel-vision about Yankees.