Friend? hah, we can do better than that! how about
Boyfriend
Special manfriend
Bestest *wink wink* friend
or Lover?
you forgot one
honey
hmm, what if Kurogane and Fai would adopt a kid thogether and the kid
looks just like one of them *child version of Kurogane or Fai...Kawaii*
and the kid starts calling Kurogane silly nicknames and Fai would be like:
"now now, you shouldn't tease your father like that, isn't that right *in teasing voice* Kurorin?"
Kurogane: "don't you start as well"! *chases Fai*Â
*syaoran coming by to visit*
Syaoran: "those two never change"