Everything
Okay, time for weird. It came from nowhere and my brain made me do it.
This is heavily based on a severely TL;DR theory of mine as to Kurama's behavior during Round 3 of the Dark Tournament. This theory states, among other things, both that he was deliberately trying to get killed, and that he had prior history with the Shinobi sect. My own interpretation of the canon, basically, because I think entirely too much. Please keep that in mind.
Think of this as a prologue for something that might turn out to be a series or even a multi-chaptered fic. The story between them just begs to be fleshed out. The pairing equals crack, to be sure . . . but I can't help it. I get slapped by my muse whenever I don't obey.
I watched him while we fought, while I dodged and danced away from his deadly-sharp shards. I watched him and I saw myself.
Perhaps I had been human for too long, for it was the human in me that I saw in this proud demon. Few demons fight for things greater than themselves, for the honor of any other being, or for a dream. It resonated, to see the cloaked pain in his eyes, veiling itself behind anger and determination, at the death of his comrade―a hurt he would likely share with no one, to the end of his days. A haughty kind of pain, like what was often in Hiei's eyes when he let down his guard; a soft certainty with it, that it was forever and there would be no easing.
So few demons ever feel that kind of pain. It is the province of those with too much humanity.
I had tried to spare him that, for the sake of a shared memory that he would never think to link with grass-green eyes. I knew this demon―had known him long ago, and his sect with him, during those times when my fame was greatest and I sometimes had need of their services. He had been younger then; I, older. He would remember if he ever heard my legend's name. I was careful not to use it.
So much regret, that I was to use him now.
But I would give him a life in trade for the life I could not spare, a moment's relief in the fulfillment of his honor. I was glad that the vessel for my decision was such as this. It would have meaning beyond itself, and that soothed my guilt―that Yuusuke and Kuwabara would never know my intent, and that Hiei would see it but would not dare interfere. He would not forgive.
Touya would finish what Gama had begun, and it would spare them―all of them―the danger of my presence. For I, the least honorable of us all, the least scrupulous and the most ruthless and the partner Hiei had chosen to avoid confronting: I was too human for this Tournament. Threatened once with danger to my mother, I had crumbled, and it had shown our enemies the way to destroy us through me. I was no longer enough of a demon to survive. Touya was the stronger, to do nothing, to avenge after, as all demons who love must.
I watched him while we fought, as the last thing I would ever watch, and waited for the moment when we would bleed together, and our efforts would save everyone I loved from my weakness. I mourned his pain. I wished him well.
And I hoped, truly, that he would never be marred as I had been.
At the end, I showed him the pain in my own eyes; he saw, and he understood; and for that moment, before the fall, we shared everything.