Clow Reed

...the one that stoped her time...

The fact that Clow and Yuuko actually knew each other comes out in the second volume, when Yuuko explains to Watanuki about Mokona Modokis and why were they created. She comes to describe the magician like this:

From this we can read that Yuuko knew well Clow and that they were probably friends of some kind, if not colleagues of magic. You must understand that this part is mostly speculation on their relationship, because there is not much explained in the story about them, just few things that makes us believe that Clow and Yuuko were close to each other and had some kind of relationship, but what kind isn’t known.

Towards the end of Tsubasa Reservoir and final moments of Yuuko’s existence in XXXHOLiC we find out that it was Clow Reed’s wish for her to open for once more her eyes that stopped her time and prevented her dying.

And when Syaoran asks if Clow wanted to bring her back to life, even if it was taboo, Yuuko answers:

There words makes us believe that the bond that was between them was something deep, but that doesn’t mean that the two of them had a romantic relationship, it could be also simple friendship, you know, best friends forever.