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:

That tricky magician in glasses who made the real version of that toy [in the picture there is Moro holding Sakura’s release wand from Card Captor Sakura]. He goes by the name Clow Reed. There is a lot of fate, or maybe I should say Karma, connected with Mr. Glasses…

Volume 2, p. 15

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.

At the moment when my life was about to expire, Clow made a wish that my life might remain and Fei Wong attempted to grant this wish with his own power; that was the beginning of it all.

- Tsubasa Reservoir Chronicle, Chapter 220, p. 10

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

Clow did nothing of the sort, just for an instant, “I wish that she could open her eyes once more.” That thought crossed his mind, but Clow’s magical power was simply too great and those strong feeling alone were enough to bring my time to a halt.

- Tsubasa Reservoir Chronicle, Chapter 220, p. 15

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.