...the known about her...
The Official XXXHOLiC Gude says on Yuuko:
The lady owner of the store that grants the wishes of its costumers, for an appropriate price. The name, she admits, is an alias. She is also known as "Dimension Witch" or "Space Time Witch". Her strange gifts earned her much recognition, both from people involved in fortune-telling and other spiritual fields, and from inhuman beings. Her age is undisclosed, but she often makes reference to anime and manga from the 1980s and earlier.
When she apears, it's in the first volume of XXXHOLiC and is extremely experienced in her job. She takes Watanuki under her wing and in an un-ortodox way becomes his mentor, and Watanuki her aprentice.
She has many personal abilities, including reading between the lines to descover the truth behind people and events, aside that she is able to contact otherwordly spirits. She also has a high tolerance of alchol, so she is know as a really havy drinker.
Her symbol is the butterfly, which represents a person's soul and transformation in Eastern philosophy (I'll talk about the myths in another part of the shrine), while her magic comes from the moon. She has a thing for alcohol (so much that she is finley attuned to the presence of alcohol in the vicinity) and also loves Watanuki's cooking, and also leading the boy in crazy situations.
Yuuko has a big sensitivity to the smmer heat, which occasionally pushes her to the brink of death.
Despite the fact that mostly we see a free-spirited and almost immature personality, when the time needs it Yuuko can show a side of her self that is almost sage-like and serious. The woman perhaps already know what will Watanuki choose in the future and loves to comment on the things that happen with the fact that nothing is coincidence and that there is only hituzen. On the other side, even if she seems torturing Watanuki and loving every minute of it, lot of times she shows that she cares for him and wants him safe.
Yuuko defenetly is one of the most powerful creatures in the XXXHOLiC (and probaly in TRC), but even if she has all this power it seems that when there is work to be done she just gives advices and then the person needs to choose alone what path to walk. I read somwhere that it seems like she is playing chess with living creatures. But to keep with her reputation as "Dimension Witch", she posseses ad uses many powerful, arcane abilities with various effects.
In chapter 214 of Tsubasa: Reservoir Chronicle, Fei Wong remarks that there was one who had their time and being severed as Sakura had, and that Syaoran's taking of Sakura's hand had caused that person to come back. Chapter 215 revealed this person to be Yuuko herself. Yuuko is revealed to have suffered a fate exactly the same as the original Sakura in that her own time had been stopped to prevent her dying. As her time begins to move, she begins to die. She breaks Sakura and Syaoran Li ("Syaoran's" parents) from the 'world' they were in (the tube in her shop), saying that she will pass on their power. The last act of her life...
In chapter 182 of XXXHOliC we find out some of the truth behind her back - she died, but Clow for a moment wished for her to open her eyes just one more time, and she did, becoming stuck in a moment, and waited for the moment that the time would run again, the moment where she would use all the power she got to help someone precous...
Unfortunatly this is the only information for now avalable on this character, and that's another thing I love her for, because we have all the things to fantasy about who she really is.