RIGHT THEN.
For those unfamiliar with xxxHOLiC, I'm going to just c/p the
Wikipedia entry about Yuuko's shop since I'm feeling lazy. /o/
The shop keeps its secrets and seems to be selective in its clientel. When Doumeki Shizuka could not enter the shop and only see Yuuko's store as a vacant lot, Yuuko explains in volume 8 of the manga that since Doumeki does not need anything from her or the shop, it does not physically or visually exist to him. As a result, he cannot enter her shop like Watanuki, and must arrange to meet her outside of it. For a shop that can grant any wish, only a few seem to have taken notice of it, though she believes it is hitsuzen that makes it possible for those few to set foot in it.
With there being no direct English translation of hitsuzen (必然), here's a quick note on it:
Can be interpreted as referring to "inevitability", destiny, or fate. Interpreting hitsuzen as "fate" is not inaccurate, but in the context of the series, "fate" is implied as something that is immutable and definite, giving the idea that whoever is bound by fate is nothing more than its puppet. Instead, hitsuzen is the idea that an action is inevitable because without it, other related events in the future could not happen. Therefore all decisions and actions are related, and there can be no coincidence, only hitsuzen.
So what does this have to do with YOU?
1. Can your character see Yuuko's shop?
If your character is the type to strive for their innermost wish or die trying, then they probably won't see her shop. Like the example above with Doumeki, even when he was told what he's trying to acheive is impossible, he didn't give up but instead tried to find his own solution. People like that don't rely on anyone else, only their own strength. I'll set her shop somewhere near the lake like in an abandoned boat shack like I had in her app or something, so if they can't see her shop, they'll just see this paint-peeling, rusted-over shack. I JUST READ THE CABIN POST her store may be the second floor of Boys' 9, we shall see! |D;;
For those who can see it, i.e. those who are LEECHES and make people do their shit for them or similar |D, they'll see this grandiose Japanese-style house (link to come later).
As for the various beasts/supernaturals/spirits folk, Yuuko often has otherworldly beings visiting her shop. They drink and carry on and mmmm snacks~ I have no problems if you want your character to hang out or do whatever at her shop, I leave it up to you. Though she'll make you clean up after yourself~
ALL THIS BEING SAID, you don't have to interact with her at her shop! If you're not sure about if you're character can see it or not or just don't want to deal with it or WHATEVER, I can have Yuuko out and about and making house-calls for wish granting. I'm pretty lax about it, and will go with whatever you decide for your characters. o/
2. Can Yuuko know about: a) your character b) your character's world?
... This question basically boils down to "can Yuuko be in your base?" XD;; Yuuko being The Dimension Witch means she has the ability to travel to/interact with other worlds and has some knowledge about them. Since she grants wishes, she also has to know what a person truly wants; what makes them tick, if you will.
I ... don't know every canon in camp! Yuuko can go anywhere from vague references of canon events or very specific details regarding your series/character. It's canon that she reads manga so she would know about the whole being-fictional thing, too, though she would also know that they really exist somewhere in another world. Anything you want to tell me would be greatly appreciated.
Going hand-in-hand with this, is it okay for Yuuko to mess with your character's minds a little by teasing, goading, whathaveyou? Yuuko is pretty blunt when she wants to be ... which is most of the time, uh. If/when the situation arises, I'll probably try to contact you and ask if messing with your character is kosher or not. Fellow CLAMPers don't really get a say in the matter, she'll do it anyway. >D
3. What is your character's wish and what are they willing to give up for it?
This question can range anywhere from their DEEPEST DESIRE to wanting their favorite dessert for dinner. I keep thinking of Dumbledore and his socks With Yuuko, every action has some kind of consequence and an equivalent price. The bigger the wish, the bigger the price. Your character doesn't necessarily have to have a wish, either! A lot of it can be situational, dependent on a lot of other factors, or really just based on what they're feeling at the time.
In knowing the wish, it's up to you if you want the wish to be granted. I'm up for wacky adventures and general shenanigans too, if you don't want anything serious. There are also some wishes that Yuuko can't grant, as well. Wishing for someone to come back from the dead is impossible for her (and she'll leave that up to the moogles anyway). For wishes like "I want to break my habit of [xxx]", Yuuko will tell your character that it's all up to them to solve it. It's all about the motivation and sincerity of your character and their desire to see their wish fulfilled.
I'M STARTING TO GET TIRED so I basically want to know if it's okay for Yuuko to grant your character's wish and to exact an appropriate price for it. I can tie this in with her camp job of Lost and Found by having your character go fetch something that someone else lost ... or something. You can tell I'm losing it here. XD;; If you don't want her to grant anything, she can say no and tell them to gtfo and do their own work.
EDIT: For a better example on the whole price thing, Tsubasa is a really good one. Princess loses her memories, boyfriend wants to get them back, has to travel to different worlds for it, HUGE WISH --> has to give up their most important things, (his) relationship with said princess and (her) memories of him, HUGE PRICE. I'm totally not pimping out my canons here, not at all.
And to wrap this tl;dr post up, one of my main goals with Yuuko is to keep open communication with everyone. I'm very aware of how much Yuuko can switch to godmode on people, and want to prevent it as much as possible. If you ever need to contact me, I'm usually on either IRC or gchat or both, and my email is erynechan [at] gmail [dot] com.
HEY CANONMATES DID I MISS ANYTHING so tired, orz