My Boy.
He left with little warning, one sunny day, with a sad little smile, a hug, and a promise of returning soon.
He'd write often at first, from several points of the world, allegedly working as a contractor, and over time he'd write less and less.
Every once in a while, Rin would come by on some business or another, and I would ask her about him. She'd always say, "Oh, he's okay, I saw him just last month!" and that would put me at ease.
I only saw him once afterwards, when he dropped by without calling first, and he was huge now, tanned and white haired, and I really couldn't recognize him at all at first. He hugged me, told me fondly "You haven't changed at all, Fuji-nee!" and we spent all day laughing, drinking and reminiscing before he left again. Only when he was gone, I realized how sad he actually had looked the whole time.
The pattern repeated itself after that, in that he'd write a fair lot at first, and then not so much, and finally he didn't write at all.
I once heard he had been executed. Another time, I was told he'd died in a warzone. I heard so many stories, and Rin never confirmed any of them.
We grew old and grey and had families of our own, and that soothed the pain of the loss, somewhat. But I never could forget my little brother, my poor lost Shirou.
My boy.