Inspired by Chapter 30 of the manga
Rin didn't really understand why after a certain age the school decided to separate Yukio and himself, but he knows that he wasn't happy about it, and that they were both actually quite upset on the first day they had to part and go to different classrooms. Before that, they did everything together. Not a day went by where Rin and Yukio were apart for more than an hour at a time, and that was pushing it.
It was also at this time that Yukio began to be bullied.
Yes, he'd been teased before, but that was all it ever got to because Rin was always there to shout down (or beat down) the ones who dared open their mouths. It had never had a chance to escalate. But then they were apart and had differing lunch and break schedules, so Yukio was much more vulnerable.
Rin can remember the very first time that he became aware of the connection that he and his twin shared—he'd been in class, sitting at his desk working on something or other, when all of a sudden he'd felt a pressure in his chest. His pencil had stilled against the paper, and he could feel his heart pounding, could hear only it in his head. With every beat he could hear his brother's squeaky six-year-old voice shouting/screaming/calling his name.
He still can't remember what happened between the moments when he realized it was Yukio's voice in his and when he next became aware of himself. All he knows is that one second he was in his classroom and the next he was behind a Quonset hut standing in front of his brother with knuckles bloodied and three young boys on the ground crying and clutching their faces. His breath was harsh against his teeth, and every muscle hurt with how tense they were being drawn. Behind him he could hear his brother crying, and anger flooded his veins again.
His teacher later explained to their father that he'd jumped up in a rage, his chair squealing against the tiled floor, and run pell-mell out the door. She'd followed him as best she could, but he'd been so fast, and she could not catch up until he had already taken down the boys who'd been assaulting his brother.
Her astonishment at how he'd known was something he never forgot.
Yukio, being the curious autodidact that he is, has read a lot concerning the psychology and biology of twins. He quickly learned that there is a lot of information out there, and not all of it is factual—what he can find a lot of is myths and superstitions. Parapsychologists and pseudo scientists seem to really love twins.
What he is really searching for is a question that everyone loves to ask, but which no one can answer: Do twins have a mental connection to one another?
Biologically speaking, Yukio realizes that since he and Rin are fraternal twins, and are no more genetically alike than any other set of siblings, that this shouldn't be the case. They did not come from one egg that suddenly decided to split in two; they were not at one time a single being. They have always had their own brains, and their own bodies, they just happened to share a womb at the same time.
Yukio has learned that logic doesn't always equal reality though.
If that were the case, then Rin should never have been able to hear him and find him when they were just six years old when he was being beaten to a pulp. There is no logical explanation (especially not when he factors in what his brother has told him about the incident).
And if he's honest with himself, he knows exactly what his brother is talking about. Even though Rin has always been able to find him when he was alone and in trouble, Yukio has always been able to feel Rin when he experiences extremes of emotion—not that he knew that at first, of course.
There was one evening when they were nine that Yukio had been in his room doing homework, and he'd been checking the clock every few minutes because it was getting late and dark and his brother still wasn't home yet. Just when he'd been about to get up and find their father he'd fallen back into his chair, breaths coming in gasps. A hand clenched the sweater he wore in fear. He'd been so scared he was having an asthma attack, or something even more serious, but he soon realized that what he was feeling felt wrong, different.
He would swear to Rin later that he could feel his heart beating like mad, but when he'd reached up to check his pulse it was slow and normal. Energy sizzled through his limbs, and he could tell that this was not fear that he was feeling (that was something he was familiar with), but anger. More than anger, it was absolute rage.
When Rin finally returned it was to meet Yukio outside on the cement steps leading up to their door. He'd known, without knowing, that Rin had gotten in some kind of trouble, had been fighting, had probably been hurt, and he had to wait and see if it were true.
He had known.
Rin can't count how many times he heard Yukio in his head, just calling his name, and then suddenly his feet carried him to where his brother was. He can't explain it, and doesn't really try to. He knows that Yukio starts reading things, but he doesn't get many answers either.
Then they started happening less and less, though, tapering off when they were probably about eleven or twelve (around the time when Yukio got a little taller, filled out a little more, and started getting recognized for how smart and talented he was). It seemed that Rin's connection and usefulness to his brother was now gone.
Yukio never stops feeling the tugs to his heart.
Rin is such an emotional cannonball that he doubts they'll ever completely go away.
It isn't all bad, though. He loves when he can feel even just his ghostly comprehensions of the absolute joy his brother sometimes experiences; but those times seem to come far and few between the older they get. No, he's much more apt to get a nice dose of rage than happiness these days. He worries about it, how angry his brother is, and how it will affect his demonic awakening (because it has to happen someday).
The day it actually does happen, Yukio swears he almost died.
He'd known something was wrong, had tried calling their father from his new room at the Academy, but hadn't gotten an answer. Earlier, something had happened to his brother, and there was an underlying buzz in Yukio's head that was not going away.
He had never felt so much fear from Rin before in his entire life. It stole the breath right out of his lungs, took the strength from his legs. Still he trudged on, worried/afraid/angry. But what he gets from Rin still doesn't prepare him for the end result.