For those interested in the main interaction between Sully and Rei; This takes place in a fantasy verse.
When they were children, Rei was captured by some very not nice humans and locked away. Because of what he is, he is branded as a monster. The logic behind his capture was to lock him up like an animal and prevent him from ever becoming problem when he got older. He was treated this way his whole life - ridiculed as a monster - even as a child that was softer and kinder with no intention to ever harm anyone, and it has largely impacted who he is today. He has become colder and a bit of an asshole, as a means to protect himself and protect others from him. It is so engrained into him that he is a monster, that he sees himself that way as well.
Sullivan was also, in his own way, dealing with something similar; ridiculed as a monster, because in this timeline intersexed individuals were treated poorly and shunned. (You can read more details on Sully’s blog about his verse.)
Sully happened upon a captive Rei one day, and luck would have it that his captors were away at that moment. Empathizing with how Rei was being treated, and thinking he had nothing to lose even if he got caught, Sully took it upon himself to free Rei. They ran away together, and became very close in the time they traveled together, bonding over being ‘different’ and how society treated them. They accepted each other from the start, even when they couldn’t accept themselves.
Sadly, Sully would awake one night to find Rei just...gone. It was blindsiding and a massive contribution to Sully having abandonment issues. As already seen in threads, Sully gets triggered and panics if he believes Rei is going to leave him again.
Though, Rei’s disappearance wasn’t without reason. A good reason, at least in Rei’s eyes.
Rei had noticed they had been being followed - followed by the very crooks that had taken Rei captive before. Rei didn’t want to cause Sully any distress, so he never told the other about it. When he was sure Sully was fast asleep, Rei slipped away and led the bad men away from Sully’s location.
Everything Rei has ever done has always been to protect Sully.
Unfortunately, Rei ended up getting captured again.
Years later, and free once again, Rei had picked up Sully’s trail. Not a day had ever gone by where Sully wasn’t on Rei’s mind.
Rei kept his distance and watched over Sully. He wanted to make sure Sully was safe, and as much as it pained him, he believed staying away was what was best for Sully.
However, Sully ended up stumbling upon Rei, unaware that the oni had been watching over him all along. Rei still continued to push Sully away, and even acted like he didn’t even know Sully. This is where their first thread starts, which you can read here.
To summarize; Rei and Sully are childhood friends and bonded over their mistreatment and labels of being ‘monsters’. Rei abandoned Sully to protect him, but found him again to keep an eye on him. Despite reuniting, it isn’t the happiest of reunions, as Rei is a colder person now, and is constantly shutting down his own feelings and pushing Sully away - all in order to protect Sully, be it from other dangers or himself, as Rei doesn’t trust himself because of what he is.
Extra Fun Fact: As children, Sully gave Rei a woven bracelet he had made, and in exchange Rei gave Sully one of his demon scales on a leather cord, made into a necklace. They both still have their mementos and Rei has never taken the bracelet off his wrist.
