WARNING: Written in My Own Heart's Blood spoilers abound. Seriously, if you haven't all of MOBY, DO NOT READ ON.
If you hang out long enough in Outlander fan groups you will hear a wide range of feelings about "secondary characters". I put that in quotes because I think calling the child of one of the main characters secondary diminishes his role in the story. Make no mistake William Ransom is important and not just to Jamie Fraser or John Grey. William is especially important in Written in My Own Hearts Blood.
Invariably, you will come across some readers who ONLY want to read about Jamie and Claire. They are only interesting in the love story. You'll hear them bemoaning the amount of time spent on other characters. "I'm so tired of hearing about William. He's such a brat. Can't we get back to Jamie and Claire?" That's fine, but I think those folks are missing the potential for a much richer reading experience.
William is the perfect example of that. At the end of An Echo in the Bone. poor Willie has the rug quite dramatically pulled out from under him when he discovers that his father isn't who he thought. In that fateful moment he went, at least in his mind, from being the Ninth Earl of Ellesmere to being just William Ransom (or maybe Fraser). He goes from being an entitled and frankly a little spoiled aristocrat to being just a guy who has to make his way in the world. Although, no one else sees it because he still has all the trappings of his title he feels it keenly and spends all of MOBY in a state of internal war between the entitled lord and the young man who still hasn't figured out who he is.
If you're reading the series only for the Jamie and Claire bits, this correlation might fly right by you, but William is an analog for the American Revolution. I know, that's a pretty big claim that only Diana Gabaldon can confirm, but hear me out.
William's circumstances have thrust him into this internal struggle between his own aristocratic roots and his identity as a man. Likewise, the situation in the colonies; distance, lack of representation, an uneven mercantile economy...leave the American colonies questioning and ultimately rejecting the benefits of being part of the British Empire. Just like William, the American colonies go from living under the yoke of aristocracy to (ideally) a meritocracy. It's a situation rife with internal struggle as history tells us.
William's situation is the same on a smaller scale. Once he no longer feels propped up by his title, he has to figure out what kind of man he is. Luckily, he's had some great teachers in the "How to be a man" department between Lord John, his uncle Pardloe and Mac, the groom. They're all leaders and men of honor. Because everyone still recognizes him as Ellesmere he's under the added pressure of being in a being in a leadership role that he doesn't feel he has earned.
Thus enter Jane and Fanny for whom William takes immediate responsibility. His exact feelings for Jane are a bit murky, but he declares the girls to be "under his protection". This should keep other men from molesting them, but I think it's a bit more than that. I think William wants to prove himself worthy of the role that he's in now that he doesn't feel entitled to it. He's testing himself whether he realizes it or not. Even though they haven't seen each other for some time, when he finds them again in Savannah, William feels responsible for Jane. It's so important to him that he willingly asks for help from the very person he wants to avoid, Jamie. When he learns he is too late to save Jane, he still sees that Fanny is looked after.
This is a common thread throughout the whole series. Leadership isn't all leading battles and collecting rents, it's a matter of responsibility to your people. Willie didn't develop this on his own. It's the same leadership that Lord John showed when he adopted Willie. We see it in Jamie pretty much whenever he's breathing. Although you could draw a direct correlation between William seeing to Jane after he death and Jamie collecting the body of Gavin Hayes, an Arsmuir man, at the beginning of Drums of Autumn. Of course, Jamie comes by it honestly. In MOBY, Brian Fraser immediately takes Roger under his wing and helps him search for Jem. We also see this with Dougal (because it all comes back to Dougal) on a personal level when he hastens Rupert's death and holds Geordie while he bleeds out after being gored.
William has learned this tradition of leadership from all of these men, even those whose examples he didn't directly observe. He has to take those lessons and earn the position that his title has put him in. At the end, he goes off to do just that, prove himself. For the recently independent colonies it took picking a fight with the biggest global bully they could find (Barbary Pirates War). Let's hope William doesn't have to prove himself on quite such a large scale. Although, knowing DG's propensity for breaking characters down to their barest elements, I fear he might.