More disjointed Capitano and Guthred ramblings are you ready for some full paragraphs:
Guthred is very good at his job, but isn't so good at being likable. He was probably more open when he was young, but his general stoicism kept getting mistaken for coldness and a lack of emotions. He didn't bother correcting the misconception because it meant he didn't have to deal with people. However, him embracing that misconception means that he eventually also bought into it, and ever since then has believed that of himself and acts accordingly. Self-fulfilling prophecy? As such, he has next to no friends and is at most civil or "friendly" with people he deems acceptable.
The Cataclysm was probably the event that pushed him to become the man we see in the AQs: "Obsessive and cruel" according to his story self. Bearing witness to the destruction of your nation was probably highly traumatizing. Guthred probably came to the conclusion that being soft couldn't get him anywhere; only results mattered, no matter the cost. After all, his home was already gone, what was another sacrifice compared to that? He was probably already slowly going that way back in Khaenri'ah. After all, the military is not a gentle place. But the Cataclysm definitely pushed him past the point of no return.
Thrain is definitely a Victorian male lead. He's a gentleman who likes to announce his intentions and is all about honour and morality. But he's not that close minded. He's very willing to take the time to listen to other people's opinions and ideas and take them into consideration, even if he ends up not doing anything different. Thrain knows that his way of doing things may not always be the best, and that is why he likes to have a second opinion. He'll be stern when the occasion calls for it but he's really just a big sap. He'll do his utmost to care for and protect anyone that's under his leadership and then the Cataclysm and Natlan happen. He knows what he wants and he'll play the long game to get it if he needs to.
Thrain likes the way Guthred's brain works. Guthred will always make his voice heard on topics he thinks are important eg. decisions impacting him/others in whichever situation he's in at that point in time. Even if it's an unpopular stance he's not afraid to voice it. That's what Thrain likes, the fearlessness with which Guthred expresses himself. He can see that Guthred is more than just the cold, stoic man people believe him to be, and the more he looks the more he likes what he sees.
Since Thrain is basically the model military man and has an amazing reputation, people are probably very likely to just accept anything he does and says. Then there's Guthred who will just criticize his decisions if the doctor doesn't think it's good enough. Guthred's way of thinking is very different compared to Thrain's, so Thrain feels like he's getting a more complete look at the situation compared to people just agreeing with everything he says. This does mean they argue a lot. Sometimes publicly. In front of the platoon and anyone else forced to bear witness.
Thrain appreciates and respects Guthred for who he is as a person, so Guthred in return opens up more to Thrain. This is when Thrain finds out Guthred can talk. A lot. Guthred will nag him for anything and everything and a normal person will get annoyed but Thrain finds it so refreshing. He's the dutiful son and soldier that everyone loves, and here comes this weirdo who just starts telling him he's doing things wrong. It's Guthred's way of caring and Thrain quickly comes to realize that.
Guthred has a jealous streak a mile wide. Pre-Cataclysm he wouldn't have done anything about it because he didn't see the point. But post-Cataclysm it would worsen to the point where it's bordering on codependency when it comes to Thrain. And then he dies and when he comes back to possess Ororon it's full blown yandere because only he can be by his commander's side.
Meanwhile Thrain's just thankful that Guthred survived the Cataclysm with him and continues to stick by him despite all odds. Then they arrive in Natlan and he notices how Guthred is quieter and keeps to himself more, but there's not much he can do about it when he's trying to keep his platoon together amidst all the chaos. Resources are nearly non-existent and morale is at rock bottom. How can he only focus on one person when he has the rest of his men to look after?
Then Guthred dies and Thrain is all alone. Ayizu is a good, trusted friend but he is not Khaenri'ahn. He is not Guthred. No one is Guthred, no one can replace Guthred. Thrain is the last of his platoon. The Night Kingdom is closed, his men can never find peace. They've suffered so much already, why must they continue to suffer even after death? He makes the admittedly rash decision to take in his men's souls. The gods have no love for Khaenri'ahns. But Thrain does. His soldiers are his, and what sort of commander would he be if he just abandoned them to their fates? He owes it to them. He owes it to Guthred.
Thrain does not truly get to mourn the deaths of his men. How does one mourn one's companions when they reside in your mechanical heart, screaming and crying at all hours of the day? At least he isn't truly alone in the world. But it is painful and he can't sleep. His mind is filled with voices that aren't his, emotions that aren't his. He has Guthred's soul in his heart but he wants him by his side, feel the warmth of a living person. He wants to hold Guthred's hand. He wants Guthred in his arms again.
All he can do now is continue forth on his self-imposed mission. No matter how long it takes, how many years, decades, or centuries, he will give to his men what they deserve.
This was supposed to be a cute post but it turned out quite serious? This was also supposed to be in point form but instead entire whole paragraphs appeared I don't know what happened. Honestly I lost my grip on this post after the third paragraph lol.
I originally wanted to talk about how Thrain is all about Guthred's tsundere gap moe..... sigh. Maybe next time. Lol.