Breaking my silence to rant about war-era nutmeg (it's like an annotated play-by-play for their parts in the war until I wrap up with some extra thoughts)
Natsume's in his first year at high school, probably his first time really being on his own in the world (he's a big mommy's boy and probably didn't really go out much before) and he doesn't really have friends. He spends his time alone in his secret room and expects to go the whole 3 years like that. Then tsumugi finds him and suddenly his childhood friend is back. Obviously, natsu tries to push him away for the sake of his own independence, but it's half-hearted and really he just can't believe he found mugi again. But he really wants to believe it so he let's mugi in (foreshadowing!!).
Meanwhile mugi has a lot of friends but he doesn't feel like any of them actually value him. Like they might talk to him or ask him for help but he thinks they have better people to hang out with and don't really need him. And then eichi comes along and tells mugi that he really does need him. He makes him feel important and like he's part of something big. So of course mugi trusts eichi and thinks they're friends. And then he finds natsu and he's like "woah! Two real friends in one week, things are looking up!" (Oh the poor bastard)
Things carry on, the eccentrics are formed, natsu and mugi get closer, and eichi puts together fine with mugi as their leader on paper. Mugi's so convinced that he's worthless to others that he even points out that eichi could just throw him away (blame everything on the old leader) and start over with a fresh slate for fine. Which is exactly what happens but mugi doesn't really think eichi would do that so he ignores the warning signs.
And then natsume finds out that tsumugi gave eichi his name. He sees what's going to happen to the eccentrics, that the student council is trying to push all of the school's faults onto them. He sees that mugi helped set this up and he couldn't feel more betrayed. He had just gotten his one friend back and thought that maybe he could start to live a little and now it's all gone. He feels broken and when mugi comes back and doesn't understand what he did wrong he shatters completely. Mugi honestly thought, at least at first, that being one of the eccentrics, being special, would make natsu happy. He thought he'd be excited to play a role in the reform of the school.
Natsu pushes mugi out, but he's not back exactly where he started. Now he has his eccentric nii-sans, which you would think would be a positive, but it really means that natsu has 4 other people that he cares about dearly and he knows something terrible is going to happen to them. He watches as they're picked off, one by one. He sees tsumugi on stage, front and center, as fine breaks valkyrie (underhandedly - they got an "unknown fan" to tamper with the sound system) and the formerly proud and undefeatable shu is left trembling on the floor.
He feels powerless but he needs to do something, so he tries to write a new ending for the eccentrics' story. He pours his heart into making a script for wataru, the last eccentric to be executed, and finishes it just in time to run over to the amphitheater. He gives it to wataru, desperately hoping that he'd read it and give them a happy ending, but wataru refuses. He doesn't even open it (in the original; in the remake he does flip through it briefly before rejecting it - not sure which is worse tbh) and rei comes and takes natsu back to the audience. He watches as mugi destroys the last of the eccentrics with his own hands.
On mugi's side, he's upset about natsu, but he really thinks that once the war is over things will be better and he'll understand. He sees the signs that fine isn't going to last, but he really really wants to believe in eichi so he ignores them all. Until the night of their last performance, when nagisa tells him to really look at his contract and he can't ignore it anymore. He finally looks and his heart is broken. All this time, eichi really was planning to throw him away after he got what he wanted. He thought they were *friends* but to eichi it was just a business arrangement. He confronts eichi backstage and begs him to tell him he's got it all wrong, that they really are friends and that the contract was a mistake or was meant to have a renewal. For eichi's part, he thought mugi had read the contract and knew this was business, not friendship. So the sudden realization that he actually had a friend this whole time and had thrown him away so easily for the sake of his dream probably broke him quite a bit, too.
Fine's last performance is awful and wataru loses on purpose to let eichi achieve his goal. Natsu has to watch from the audience and mugi has to perform on the stage without letting his broken heart show. Everyone's having a bad time! (Except wataru, he's having fun playing a role, though I suspect it hurt him to deny natsu). The eccentrics are defeated, the student council has made its reforms, countless students have been forced out of the school or killed themselves due to the new pressure, fine disbands and natsu and mugi are alone again.
Mugi is doing his last task for eichi and burning the incriminating files from the war when natsu appears. They're both lost and hurt right now, and they each feel the other's pain. Natsu still feels betrayed by mugi, but now he sees that mugi was also betrayed and he can't help but sympathize (internally ofc he'd die before he said that). He brings his script to burn, but before he puts it in the fire he asks mugi if he thinks this would ever happen again. Mugi says no and natsu burns the script. But mugi doesn't regret what he did. He regrets the pain he caused natsu, but if he could go back in time he would still help eichi. He feels that what he did was for the greater good, and that it was necessary. Natsu is still bitter, but I suspect that mugi's honesty is more appealing to him than eichi's or Keito's regret.
So when mugi asks natsu to be in a unit with him, he's shocked at mugi's boldness after he just betrayed natsu, but he also doesn't want to give up on the strongest connection he's ever had. He "reluctantly" agrees (hiding it ofc by claiming he's only doing it to keep his enemy close) and they vow to stay together (till death do us part). And that's the start of switch!
It's the regret thing that I keep coming back to. Mugi does understand what he did to hurt natsu, even if he pretends he doesn't, but he feels it was a necessary evil. He's more of a repent/beg for forgiveness type than a regret/try to change the past type. Natsu does have regrets about the past, though. Even after burning the script, he doesn't want to let go of the eccentrics' story. He's still trying to give them a happy ending years later as he tries to take over the school before his graduation. He doesn't want to see another war and more crying kids, and he thinks, subconsciously at least, that by preventing bloodshed in the future he can wipe away his own tears in the past. They're two sides of a coin, one moving through the future while remembering and repenting for the past, the other trying to reach back to the past by changing the future. And in the middle is sora, keeping them from straying from each other and anchoring them in the present (this isn't about sora but he's my favorite so I will bring him up teehee)
Anyways uhhh I'm sure nobody read all that but tldr is nutmeg good 👍