Both of these gents are friends and rivals of our protagonist, Frederick, competing for some of the same women and learning tactics from each other. They're super different—Arnoux is the ultimate businessman who's obsessed with money and Deslauriers is an absolute Republican. So while these two are antagonists to Frederick, getting in the way of his pursuits, they're also antagonistic toward each other. Minds blown.
Yep, that's right. We're going to say that Frederick might just be his own worst enemy. An antagonist, after all, is someone or something who gets in the way of the protagonist, right? And who gets in Frederick's way more than Frederick himself? If the guy would just wake up and do something, he might be able to realize some his idealistic dreams. Just sayin'.