Yay, I don't know.
He watched the little twins, walking through the forest. Such foolish little creatures, so young and vulnerable, yet they walk through the woods at night. How old were they? Five? Six? Hmm, not that it much mattered. They would not last much longer, proving their ill-fated choices.
And prove it He did. The larger of the look-alikes died first, shoving the other as far from him as he could. So brave for being so young, but it would do nothing to save him or the other. Once he was "disposed" of, the faceless entity turned to the other, noticing the features of the small youth. The colorless skin, the pale grey eyes, the shock of unruly black hair, stuck in the jaggers he'd fallen into when pushed. The Slenderman had never felt so conflicted with what to do with a child. Every instinct within Him disagreed with his other instinct to kill the pitiful form before him. Maybe it was the hatred and anger gleaming behind tears, or it was the defiance he bled from every crevice of his mind. Or maybe it was just that. The mind that was a steel trap, locking out his influence like a vice, though it was strained from being used for so long. For being a lowly mortal, it was such an extrodinary being. The Slenderman wouldn't hurt it, he'd give it guidance, watch over it, care for it. But he'd always need to track it for that to be possible.
The child grit his teeth, wincing and glaring angrily as though a rabid wolf as He grabbed an inch or so above the small elbow. After a minute or so, the faceless being removed his hand to reveal a black mark burned into the small beings arm.
A simple O with an X within.
He'd marked the child as one of his own.
He'd have a proxy to train when they'd grown older.