Fai D. Flowright and Kurogane
...the magician and the warrior...
There is not much to write about the connection Fai and Kurogane has with the witch.
Kurogane was sent to her so he can learn to be more humble and less angry, and his wish is to go back to his country and to protect his princess. He is not really taken by the witch, still she does mock him for his attitude. His price is his most dear possession; his sword which he reluctantly gives away to her and tells her that he will be back to get her.
I think the both of them actually like each other; but it’s like Kurogane’s relationship with Fai, constant anger and mockery, which amuses Yuuko to no end.
The Fai side of the story is a little different; I think their is based on mutual respect and the big powers they both wield, so much that she was in another dimension created pocket to not trigger the curse that was on Fai.
Fai came to see her for the opposite wish from Kurogane’s and that was never to get back to the country of Celes. But the price to be payed was to great and he simply understood that and decided to travel with the group he soon found to love.
The magic those two wield makes that they understand each other better than anyone else probably and also they understand what the real sacrifice is when you cannot do differently then follow the flow making them a little closer without much interactions.