Jonathan Wolverton Randall is the masochistic British captain who torments Jamie and Claire every step of the way. He's the first man Claire encounters in the past, and the fact that he's the spitting image of her husband, Frank (Jonathan is Frank's great-great-great-lots-of-greats grandfather), doesn't make things easier for her. She's tormented by his resemblance to her beloved Frank, observing:
I had loved a smile like [Jonathan Randall's], and that dreamy look had roused me in anticipation. Now it sickened me. (35.213)
Bummer, right? Randall eventually captures and rapes Jamie before being presumed dead in a stampede of cattle (seriously).
Randall goes to great lengths to get what he wants, and he spares no expense with cruelty. Don't forget that he's called Black Jack, a name Jamie explains he's earned "with reference to the color of his soul" (16.227). Consider yourselves warned.
When it comes to Claire, Randall wants to know who she really is. He punches her (and then says, "I trust you are not with child, Madam. […] because if you are, you won't be for long" (12.124)), cuts her, and threatens to rape her. None of these tactics work, though, so he just keeps getting more extreme. Randall can't admit that he's met his match with Claire, who is wittier than he, and a woman at that, which makes her refusal to submit to him extra insulting (as far as he's concerned, anyway).
With Jamie, to put it bluntly, Randall wants his sweet Scottish rear end. He uses the same techniques on the dreamy warrior as he does with Claire—physical abuse, cutting, branding with a hot ring, and actual rape—which makes it clear that this is Randall's preferred way of doing things. In fact, pretty much every scar on Jamie's body, from his mangled hand to his entire back, is the result of Randall's handiwork.
We lose count of how many times Randall tries to rape Claire (in our defense, his efforts begin the very first time he meets her in the woods) and how many times he does rape Jamie. We are certain of one thing, though: Dude gets off on pain. As Claire realizes during one of her many unpleasant encounters with Randall, "He wasn't going to enjoy it unless I screamed" (21.78). And then she wisely keeps her mouth shut.
The hardest thing for Jamie is that Randall almost rapes him lovingly, if such a thing is even possible (it's not). He whispers sweet nothings into Jamie's ear and, Jamie hates to admit, even succeeds in arousing him, despite how much he doesn't want it. Although Jamie calls this Randall's version of love, we're not sure the man is capable of such an emotion.