Amairin AU wip pasted from my notes
in which Amaimon and Rin are soulmates and the princes and Lucifer know about it but Rin doesn't. Therefore, Mephisto's playing matchmaker and trying to get Amaimon to play with Rin.
This is non traditional abo, demons are either submissive or dominant. They are all born one or the other and this decides what role they play in a mating/marriage as far as procreation goes. Soulmates are a rare occurrence in mid and low level demons and uncommon in higher level ones so they simply choose their mates for the most part but soulmates are definite for Princes/Kings of Gehenna.
Note/context for later: Through this, Bon and Rin are hinted to be crushing on one another. Rin had initially thought he was cool and wanted to talk to him but was turned off for a while by his shitty personality ofc. He’s still Rin so he still attempted to be civil (but Bon’s always starting shit) and he cares for everyone, no matter how shitty, so he still saved him from the leaper. After they made up, Rin absolutely went back to fawning over him and trying to insert himself into their friend group as he does in canon cause he’s cute. Bon absolutely got a crush on Rin after Rin jumped into the mouth of a demon without thinking like the fearless idiot he is and then turned around and yelled in his face that, not only did he take Bon’s aspirations seriously but that he has the same one and was going to fight him about it. Shima and Koneko picked up on this pretty quickly of course and this led to them encouraging Rin’s antics and inviting him into their group in every way they could like in canon. In between the moments summarized in this doc, there’s definitely romantic moments between the two in which they’re absolutely fascinated with the other person, though that’s not much different from canon either.
As far as romance with Shiemi, it follows the manga more and paints them platonically. Rin didn’t know exactly whether he liked girls or not but he did sense that Yukio liked Shiemi and she basically immediately was friendzoned in his mind bc he’s not about to steal his brothers one and only love interest he’s ever seen thus far.
Originally, Amaimon didn't know but was told during the first meeting with Mephisto. He had suspected and, rather than in canon, he had hesitated to ask if Rin was his mate. Mephisto confirmed and took Amaimon asking as a hint of interest, which he interpreted as an invitation to set them up cause he's Mephisto.
Amaimon suspected this because the kings of Gehenna are all mated to one another. The unmated ones that remained were Mephisto and Amaimon and Mephisto had laid no claim on Rin.
It's worth mentioning that from this point on, Mephisto wished to use Amaimon as a pawn in his plan. He knew he could control Amaimon and since Amai was the dominant mate, he could control Rin further in return.
At the second appearance, when Amaimon is watching Rin tame Kuro, he expresses that he'd like to play with Rin to assess his ability as a mate. With demons, a big part of the mating ritual is to fight their mate and test their strength, to decide whether they are worthy. This is not really needed for soulmates but for traditions' sake, soulmates tend to still participate. In Amaimon's case, there was the added fact that he only intended to claim Rin if Rin proved worthy, soulmates or not.
Amaimon then decided to get his curiosity of Japan out cause it had been killing him and he wished to learn a bit more about the culture Rin grew up in. He wished to study it to possibly understand Rin better and analyze him accordingly when they meet, as Mephisto had warned him that Rin had not experienced any kind of demon culture or education in his life and was rather dumb to the subject.
Once he'd finally finished his spree and laid his collection out in Mephisto's office, he immediately asked to go play with Rin as he thought he was now ready. When he got to the amusement park, he'd watched with irritation and little patience as Rin floundered around a human girl, but chose to follow Mephisto's orders and wait to get him alone. He was curious about the fact that he couldn't smell Rin's demonic aura, the only way he'd detect if he was, in fact, his mate. He wondered idly if Mephisto was only mistaken and if Rin had actually inherited the demon mating hierarchy or not. He realized once he opened the Koma sword that Rin was, in fact, the most amazing thing he'd ever smelled in his life and that his demon heart was sealed inside the sword, along with his submissive status. Rin on the other hand, could smell something about Amaimon that was gnawing at him but without the context of why, he mostly continued as he would if anyone were fighting him like that.
He kicked Rin around for a bit and was let down, his interest waning despite his instincts screaming at him. He started to get angry at the situation and this pushed him to break Mephisto's rules. Again, soulmate or not, he didn't want a weak mate. His interest came back tenfold when Rin had finally started to fight back, meaning his irritation was far greater when Rin had saved Shiemi and calmed his emotions again. He wished to rile Rin up once more and see his beautiful flames again but Shura interrupted. In the end, he had decided to back down and continue their fight in a more suitable time and place.
Shura takes Rin to the base and Mephisto is pleased when he can smell the territorial scent marking from Amai left all over Rin and the Koma sword. He chose not to reveal the purpose of Amaimon’s visit to anyone just yet of course, but couldn’t help but feel a bit giddy at the imposing scent of possessiveness and lust that clouded Rin’s true scent. It was overpowering since Rin’s true scent was sealed in the sword at the time so he couldn’t help but laugh and subtly cover his nose at the ridiculousness of it in the moment.
Afterwards, Rin had idly mentioned to Kuro that Amaimon had wanted to "play" and that he'd had a sick version of playing that meant fighting. Kuro cleared up that fighting is playing for demons, pointing out that Rin himself gets excited during fights and Rin realizes he had initially been excited to fight Amaimon. This is what led to the scene on the roof in which Rin and Kuro play together.
Alternatively, following the manga, Rin couldn’t sleep because he felt feverish and restless and wanted to take Kuro to play to let out some of his nervous energy. He figured it was emotional strain. While he’s talking with Kuro and Kuro asks to play, it reminds him of Amaimon saying he wanted to play earlier and he mentions that it was really weird to him. Kuro then tells him it’s very common for demons to fight one another as play, the way wild animals will play-fight to train their bodies in a fun way while they have free time.
On the note of Rin being tired and sleeping through half of the next day, it wasn’t actually all due to them playing all night but late that night and into the next day, Rin had run a pretty high fever that left him a little queasy. This was why he started feeling restless and nervous at first when he originally got up and went to the roof with Kuro. While they were playing, his energy was sapped pretty quickly and he felt drowsy and hot and had gone back to bed. He ended up sleeping all night and well into the day bc of it. This was part of the reason he actually fucked up once he got to Shura’s class but mostly cause the boy can’t read for shit.
Between this scene and the next chapter, add in Mephisto and Amaimon one on one. Basically, Mephisto had gotten wind of Rin’s absence in the morning and made a hypothesis that, at about that time, Rin would be experiencing a ‘phantom heat’ after meeting his soulmate and not being marked right away. It was a common occurrence and while it goes away fairly quickly on its own, it would make Rin a bit drowsy and feverish for the next couple days and his body would let off pheromones akin to a true heat and distress in an attempt to draw his mate back to him. Basically, he’d been recounting this to Amaimon to remind Amaimon that, no matter how much he’d want to give into the scent throughout this duration, he’d have to stay put and wait for the next most opportune time to see Rin. Of course, Amaimon had already been throwing a bit of a fit about not being able to finish playing with Rin but he became more pissy in his own shithead way about not being able to go see Rin right away, even though he was able to smell his mate from across the campus already.
Adding on, Shura had been so calm about his tardiness because Yukio had given her a heads up that he had a bad fever and really was so deeply asleep that he wasn’t waking up for anything that morning. Shura was suspicious and wondered what could cause it, since demons don’t get colds or anything, but with soulmates being such a rare thing in demon society, there’d be no way for her to understand what was happening to him. Even the most knowledgeable humans among the Vatican had no idea about soulmates, and really only vaguely about demon mating; that there were mating seasons every year and a half or so in Assiah and that reproduction was similar to a human’s. They’d simply assumed that this meant Gehenna was split into males and females (only sex wise, of course) the way Assiah was.
He finally told Rin they were mates at the next fight on their camping trip. Before that though, when he was setting up to face Rin again, first of all he’d gone on a rampage that was probably more aggravated than it needed to be while ‘greeting the spirit of the forest’. Though Rin’s phantom heat had passed, Amaimon was quicker to tune into Rin’s human scent, which was disgustingly muddled with the other humans he’d been too close for comfort with. He’d been focusing on it quite hard, leading to him shutting down and ignoring Mephisto, who finally started to grow a bit worried that messing with soulmates may have been something too hard to control, even for the prodigal son himself.
In the meantime, Amaimon grew irritated watching the students bring back the peg lantern. One, because Rin was in the center of his wretched group of humans. Two, because he them had to wade through the river of demon worms without his scent marking or mating mark that told them to keep their grubby little legs off of Rin. Lastly though, because the peg lantern itself reacted more to Rin than the girl as a submissive and chased after him as if the lowly demon could ever possibly deserve such a mate. His little shitfit aside, Mephisto kept him under control for the time being as he teemed with the anxious energy of a caged bear. The giant moth actually made to grab Rin at the end to retrieve him for Amai but the poor boy had to wait a bit longer cause Rin’s friend saved his ass. Once Rin’s finished his assignment Amaimon loses his patience and pops out on the forest to fuck shit up.
He'd taken Shiemi hostage under the impression that Rin had a crush on her, taunting Rin with the whole marriage thing. Ofc Rin was just confused as hell and wanted Amai to put his best friend down. It wasn't til Amaimon was hit with the fireworks that he realized it was Bon, not Shiemi, that Rin was interested in. That's why he went for Bon instead of the one that actually laughed at him. Bon started yelling at Rin and ignoring Amaimon and ofc Amai was even more pissed off that this asshole had the audacity to not only go after Rin but to straight up ignore him, Rin’s true mate. Rin yelled to stop and as much as Amai wanted to kill Bon right there regardless, he saw Rin grab his sword and realized he was getting exactly what he’d wanted, a chance to complete the mating ritual and take Rin.
Rin draws his sword, Bon realizes everything’s a lie, and Amaimon and Rin finally start their fight. Amaimon starts announcing the truth to Rin, about what they are to each other, that Amaimon found him truly worthy, proclaiming his flames were beautiful. Ofc Amaimon got excited again and they started to fight like in canon and lowkey he was getting excited in another way because in his eyes, they had started the most intense and beautiful mating ritual the world had ever seen while Rin just wanted him to die. Amaimon grabbed a hold of him from behind and Marked his neck, making Rin panic and kick him, stabbing him and trying to burn him after. When Amaimon grabbed Rin’s tail, an emerald ring of some sort appeared fastened to the base of Rin’s tail. He’d screamed at the sharp pain of it and Mephisto had assumed it was simply because Rin had never had his tail tugged in such a violent way. It was only afterwards that Mephisto realized the true depth of the situation, which made things a lot harder for him.
Once he was marked, Rin had been overtaken by a demon heat. Before Amaimon got ahead of himself and took Rin right then and there, Mephisto grabbed the two, thinking he could stop them there and use Amaimon as he planned.
Then, Rin split the clock in search of his mate and Mephisto was like oh shit. When Rin is thrown into view of his classmates, he's lost to emotions and instincts. He still has no idea why he feels like shit, his mind is swimming and on attack mode and he's trying to slow his mind down. He attempted to run toward Yukio, grasping for protection and touch from someone he loves. Yukio goes to shoot him, Shiemi runs out like in canon and calms him down with a hug. His body reacts and calms down because she's registering as a friend and another submissive type, someone who won't put him in danger and only wants to help. (In demon culture, a close submissive friend is usually in charge of comforting and taking care of the submissive demon in heat between waves so that the dominant mate can recharge as well.) Once he was calm, the exhaustion hit him and he passed out, being dragged to court with the others.
Meph had genuinely meant to keep Amaimon contained, he locked him away and came to meet the others, then went to the court. When Rin started having his nightmare, Amaimon had felt the discomfort through their new bond and, already half feral with what had happened, he broke free and headed toward Rin in a rage. This is why he fucking destroyed everything and started yelling about being able to smell Rin. At this point, everything is fucked, Mephisto has lost control and has to rely on Rin being confused enough that he continues to fight Amaimon, or that Rin and Amaimon can escape to Gehenna fast enough that they don't get their asses kicked.
The next part is planned but gets dirty so it’s upon request 👀