This isn't that fic, but what came out when I tried to write it.
Title: Mobius Paths
Fandom: DC AU
Summary: Sometimes there are many ways to save somebody. And sometimes there are none.
You walked away. The boy was provided for; he only needed family and time. Neither was in your power to give, so you walked away.
But you kept vigilance over his nights from wherever you went. That much you could do. You shared his single, recurrent nightmare, his all-too familiar loneliness. You watched from afar -and sometimes from up close- as he turned pain into purpose and himself into a tool. It was both an awful and an inspiring thing to see, as so much of what humans did.
You went back to Gotham City the night after he did, just in time to see him bleeding in his father's studio, all alone in his dusty home. His body dying of shock, his mind closing upon itself in grief.
Just in time. You flew through a window, saw yourself through his eyes in the shape death and salvation took in his mind.
Something happened inside him and he decided to live. You hoped it was the right decision.
"Who?," he asked weakly.
"It's me, sir." Alfred Pennyworth had died in an undeclared war years before, but he had been a good man, all things considered. Taking his name and form was an homage, and modifying Bruce's memories so he had always been there was perhaps a way to grant Pennyworth the respite his deadly profession had denied him.
"I thought... I thought I was alone in the Manor." His extraordinary mind still resisted yours, but the lack of blood made it a losing battle.
"You are not alone, sir," you said as you began to clean and bandage his wounds.
"I saw a bat."
There was an ending in his words, and a beginning, and many other things you weren't sure you understood.
But you didn't walk away.
The funny thing about this is that it explains a bit one of the little mysteries of Bruce's pre-Batman period, namely "what the hell was Alfred thinking?"
There was no Alfred, and to J'onn the thing made as much sense as anything else humans did.