So, I'm rewatching the series, and I was overcome with the urge to reread The Beginning. Sadly, however, I can't find the link -- I thought I'd saved it, but no dice, it seems. I can't imagine why not.
In any case, this fic is between the time when Yusuke fights Kurama and Hiei, and the beginning of the Four Saint Beasts arc. It's a K/H slash fic, and basically the two of them are getting to know each other on a journey the Koenma sends them on. This is a multichapter fic, and the author inserted drawings into it. At one point, they separate, but Hiei comes back. Also, for no particular reason, Hiei is part dragon, I'm pretty sure.
If anyone could send me a link, I'd be eternally grateful.