Yukio may have been the teacher – the wiser, calmer, and more accomplished of the two, but he never placed himself any distance in front of his brother. He never aspired to exceed his brother. From their childhood of always being together, Yukio had accepted that he was the little brother, and that his Nii-san would always be with him. And that was just fine.
At the Academy, Yukio held the outward appearance of a mature, resigned babysitter to Rin. Contrarily, he guarded his position of "little brother" with certain selfishness and pride. He was the closest to his older brother. He was and would be the first in Rin's thoughts. No one else was allowed to be closer, not Shiemi-san, not Ryuji-san.
When Yukio was much younger, he had once gotten in a dispute with Rin over some trivial thing. They had sworn to never speak to each other again, but the silence was broken within an hour. Father eventually found them both curled up on a couch, asleep from fatigue.
As the childish phase passed, Yukio argued less and less with his brother. Winter melted into spring and bloomed into summer and fell into autumn. He gradually realized with the changing of seasons that they were different; they were meant to lead separate lives, and that was something decreed from birth. Rin's fate would eventually tear them apart, and nothing Yukio did could ever change that fact. Consequently, Yukio vowed to protect their relationship no matter what. If it meant keeping a secret from Rin, then so be it.
If it meant becoming an exorcist, then so be it.
As long as his older brother can smile and call him "otouto," anything was fine.
When he saw their Father's mangled body on the floor, cold and lifeless, he had tried to hate Rin. He tried to blame Rin for Father's death. He tried to convince himself that, no matter how much his brother acted human, Rin was still a cold-blooded demon inside. Yukio could not imagine the agony of losing someone—Rin—so precious—dear, dear brother—ever again.
Only during their confrontation in the empty classroom
Don't you ever point a goddamn gun at your brother again.
did it occur to him
We're family, dammit!
that he would never have been able to hate his brother.
On rare occasions Yukio would wake in the middle of the night to find himself sobbing, and Rin would wordlessly pull him into a crushing embrace. On those nights he clung to his brother's shirt for dear life, much like when they were kids. Yukio would sob into the crook of Rin's neck as his brother whispered into his ear, "I'm here, I'm here… I'll always be here…"
And in the dark privacy of their room, Yukio would always believe him.
Oneshot-ish drabble. Con-crit appreciated! The brothers' relationship is one of my favorites, of all anime. Enjoy!