One of the big theories flying around about the origin of the Troubles, or at least of AudSarLu's role in it, is that it started out involving her and two men. We've seen this theory in one form or another in a couple of different places, and we've put it forward ourselves given the parallels between Duke and Nathan, and Vince and Dave. In Lucy's era it might have been James and Garland, as the two male figures who were closest to her as far as we know, but we know so little about Lucy's time that it's hard to make anything out of it more solid than a guess. That said, the overwhelming body of evidence is in favor that something was going on with Lucy and two men, in a similar way to something going on between Sarah and the Teagues, and now Audrey and Duke and Nathan. And it is the general opinion of this esteemed pair of ladies that the show is working both the rivalry and the camaraderie in balance to resolve Haven's Troubles. If love is the answer, as described in the season three finale, then any form of love tying Nathan and Duke together despite or because of their differences may be the clue to fixing Haven that's been missing all these years.
So! Starting out, we're introduced to Nathan first as a soft-spoken cop, gentle and relaxed but also serious and dedicated. He cracks jokes with Audrey as he saves her from toppling off a cliff, but he also clearly takes his duties and responsibilities seriously. And Duke also, in fact, cracks jokes after saving her from drowning after being knocked unconscious into the water, meaning that both men are introduced in the pilot episode as being fun guys to be around who will, without hesitation, save the life of a stranger. We're also introduced to the conflict between them, what with Nathan describing Duke as "all bad" since he was five years old, and saying that "everything about this guy is a pain in my ass." Poor Duke. Still, this portrayal isn't reinforced by the narrative, as Duke is depicted by the writers, director, and actor to be more of a gentleman rogue than an actual asshole.
Shortly after that, in fact, immediately after that the conflict between them gets dark. We have comments implying that Nathan is less than upstanding by "it's funny how you get a badge, you get to harass me" (Harmony 1x03) and Nathan referring while chained up and crazy to "the things you've done. The things you still do. To me." Note that we never, ever get an indication of what that's about. There are hints and a couple of anecdotes about what they got up to as kids, mostly Duke harassing Nathan, but there's only one specific incident to which Nathan might be referring, and that is implied to have been a couple of years ago, so less likely to be referred to as "things you still do" (although it's possible Nathan holds the grudge that close and immediate). Still, both of them are already either protective (Nathan) or defensive (Duke) of Audrey, setting the stage for the interactions between the three of them to come.
Over the next couple of episodes we have more sympathetic views of Duke, first with Audrey demonstrating faith in Duke and showing compassion and that she's been paying attention to him with how she handles him and the restaurant and his friends. Even Nathan shows some sympathy to Duke over the death of his friend, demonstrating that when there is genuine and serious hurt involved he can put aside his grudge at least for a little bit. This is underscored in the very next episode when Duke is in real life-threatening trouble and Nathan stops being nasty and turns quietly concerned, even caring despite the fact that Nathan more commonly gets abrupt when agitated, e.g. "You okay? Need a hand?" "When you're nice to me it reminds me that I'm dying." "Well hurry the hell up then." "Thank you." When Audrey goes to give Duke the baby, when Duke is most visibly and rapidly dying, Nathan barely looks up from Duke. Stays talking to him and keeping an eye on him while Audrey takes the baby to a greater distance to restore at least some of Duke's health and youth. For a guy who was introduced and demonstrated to be on the edge of bending or breaking the law just to be a pain in Duke's ass, that's a strong indicator that more positive feelings also exist somewhere deep down. The cherry on the sundae of emotional awkwardness comes at the end with a scene that follows romance movie blocking almost to the letter. Duke has a moment of needing to process he has a daughter he can never meet by a woman he never knew, and he sort of totters to the door. Opens it. Nathan's there with the earnest, solemn puppy look aimed right at Duke. Duke sighs and closes the door and the music swells and I'm sure this scene could be more textbook but it's hard to think of how. The emotional implications fall somewhere between the murky middle of the romance movie when things are at their bleakest and the glimpses at the beginning of each half of the couple as they realize there might be a connection to be had. And while I'm not saying the writers or director necessarily intended this to imply a romance, it pushes all the right buttons to telegraph these two people have a deep connection.
The comment from Nathan that "when you die it's gonna be 'cause I killed you myself" might be foreshadowing, given Nathan getting that tattoo later, but after the stress of Duke's near death experience (Ball and Chain, 1x04) things seem to go back to normal for several episodes. Nathan and Duke go directly for each other's throats when locked up on a remote island with Audrey, a few sort-of friends, and a potentially homicidal Troubled person who could be impersonating any one of the group. And on the other hand, when the threat clearly comes from outside in the very next episode, they move in concert quickly and without thinking. This is also the first hints we get as to Duke's inevitable death and the first intonation of the chorus we've been getting over and over again in the past couple of seasons, the refrain of "screw fate." Through the end of the season and into the second season we continue seeing them work together when the danger is imminent and comes from outside, and bicker and snap at each other when it isn't. Audrey even recruits Duke as backup after learning Nathan will be threatened by the death of the firstborn plague effect of the most recently manifested Trouble, to which Nathan raises no objection. Romantically, Duke's attentions are taken up for most of the season by his conflicting feelings for his estranged wife Evi, leaving Nathan with no cause for aggression towards him that way and also giving us a good idea that if Nathan objects to Duke as a romantic partner for Audrey, it's only because Duke is Duke and they have history. Nathan clearly feels shut out (Roots 2x05) when Audrey takes up with Chris Brody, but he's still able to share quite a bit of camaraderie with both Chris, as long as he doesn't look at him, and even Duke, and all three of them even have a moment of buddy teasing and bonding. (Audrey Parker's Day Off 2x06) It could even be argued that taking romance with Audrey off the table for both of them helps Duke and Nathan's relationship. While there's some conflict in Friend or Faux and Lockdown (2x08 and 2x09) it never escalates to the degree of refusing to deal with the other person that it did in the first season. There is, in fact, more conflict between Audrey and Duke due to the activities of Reverend Driscoll than there is between Nathan and Duke; Driscoll seems to act repeatedly as a source of tension between Audrey and her boys. As far as Nathan and Duke are concerned, matters don't escalate again until the very end of the second season, when Nathan believes Duke has kidnapped or in some other way harmed Audrey. He breaks into Duke's boat and is clearly prepared to kill him if, something. It's not exactly clear whether that's if Duke doesn't let Audrey go, if Duke doesn't give him the answers he wants, if Duke makes some wrong move, or something. Duke has to physically stop Nathan from shooting him, although the intervention of the Toomey Trouble does help.
There's fewer of the bonding moments in the third season than in the second, although there are also strong gestures that way as well. Nathan calling Audrey out on using Duke (The Farmer 1x03/Over My Head 1x04) comes to mind, showing more compassion than he usually does for Crocker. Duke's reaction to Nathan's death and his willingness to do whatever it takes to get Nathan back, too, including the copious use of "my friend." Towards the end we have a touching bit of a moment, with youthened Duke and Nathan discussing how they would deal with being around Audrey's age, again, if Audrey left and came back and they had to do it all over again. Neither of them get possessive or jealous, even though realistically Duke would have the better shot if he did stay youthened, being as he'd end up closer to her age. It just doesn't come up, and with all the discussing of high school Nathan might also be coming to accept that they were young and stupid, and Duke's not always harmless pranks when they were young shouldn't be taken as commentaries on his character that stay the same through all the intervening years. All of this, culminating in Nathan asking Duke to take care of/go get Audrey while she's in the barn, despite the fact that he wants her out or himself in there with her more than anything.
Season three also sees the escalation of the romantic tangle between Audrey and Nathan and Audrey and Duke. Starting from the first episode of that season where Audrey is reunited with Nathan and Duke, she goes to Nathan first and the clear implication is that Duke feels shut out because of the implied romantic relationship there. This continues through to when Audrey discovers her deadline and attempts to push both men away; Duke refuses to accept that this is what she really wants, while Nathan is hurt and takes her rejection at face value, creating a rift between them. We've already discussed and laid out our evidence for this in our recaplyses: over and over again in the third season Duke demonstrates, with that one exception, that he's willing to accept Audrey's decision, presumably as long as he's sure it is her decision and not something she's attempting to convince herself is the right thing to do. Nathan, meanwhile, follows her expressed desires which she herself may not have decided to follow because of one thing or another; in the most glaring example he continues to seek out a way for her to stay even after she agrees to go into the barn and continue the cycle. The end result of all of this is that we get, over and over again, romantic near-misses with Duke leaving him rueful and longing, romantic near-misses with Nathan due to one or both of them getting skittish or slipping up, and the center of Duke and Nathan's conflict shifting consistently to Audrey. Nathan's animosity towards Duke becomes less and less about all the illegal and morally dubious things Duke's done, and more and more about the fact that either he's bad for Audrey or he's taking Audrey away from Nathan, depending on how much he's lying to himself at the time. The aspect of Duke taking Audrey away from Nathan comes out particularly because while Duke constantly hopes they can find a way for Audrey to stay, he is much more accepting of her departure than Nathan is, therefore at times Nathan seems to believe Duke wants her to go away. I won't go into how this mirrors Vince and Dave's argument in the season three finale. I'll just leave this here so you can chew on that for yourselves.
Duke and Nathan both have deep-seated abandonment issues, too, making their focus on Audrey and her imminent disappearance affect their behavior even more. In Duke's case, either his family passes out of his life leaving him free to take account of his own affairs, or the few women he's become close to and come to trust in his life leave him (Julia, Evi). Generally, they seem to be leaving because of their perceptions of who he's become, in Julia's case because she straightforwardly doesn't like what he's doing, and in Evi's case because she takes questionable actions to solve what she sees as his ongoing problems. Duke doesn't have any more family members he's close to, but Audrey's imminent departure has to hit close to home. Meanwhile Nathan has watched Haven's dysfunction swallow up everyone he ever cared about, first his mother who seems to have died around the time of the '83 Troubles cycle and then his adoptive father, and now Haven's going to take Audrey from him too. With less of a rivalry they could use this as a bonding point, a means to reach out to each other and find support in their shared loneliness or need, but between the two of them Duke doesn't seem to be able to find a consistent method of reaching out to Nathan, and Nathan's prickliness prevents him from reaching out to Duke. And in the end it's all moot anyway, as Duke is now in the barn and Nathan is entirely alone. It's worth noting, however, that in the end they did recognize kinship in each other, so with luck that will translate into greater solidarity, trust, and affection in future seasons. With a whole lot of luck.
And of course if they manage to achieve that it might help FIX HAVEN. We strongly suspect here that one of the reasons they've been highlighting the intimacy of the relationships between the trio in all permutations, not just romantic intimacy but intimacy and trust in general, is because of the "love conquers all" hint. In the first season their relationship was conflicted, full of mistrust and latent violence; the second season had them at times broaching the idea of truce and, certainly, learning more about themselves and each otehr. But it isn't until the third season when they both join forces to help Audrey that they manage to find some sort of balance, romantic implications with Audrey included. Coincidentally, this is also the moment when things become really dire in Haven, with meteors still falling and the barn and its lying avatar who lies. Of course, it's only coincidental if you believe in such things. Which we don't. Omnia vincit amor, remember?