Her two worlds
Almost from the first volumes we can find out thaat she is an important character
for both series. While she apears more in the xxxHolic series, at least at the
begining, towards the end her apearences in Tsubasa: Reservoir Chronicle are
quite lot, aside that she gives the major explanations of why everything happened.
Still during the Tsubasa: Reservoir Chronicle series, everytime the group needs
advice or something, they use Mokona Soel to contact her.
For a moment we can’t say we have two diferent series, because the wo main characters (Yuuko and Watanuki) are in close relationship with the two of main characters from the other series (Sharoan and Sakura).
As said, Yuuko is important for both series, she begins in xxxHolic, continues in Tsubasa: Reservoir Chronicle , to vanish in both of them.
I can say that at the beginning I didn't pay to much mind on the connections between both series, probably because the CLAMP don't give lot of hints about it, just the »connecting via mokonas«. She does know they are coming, and is prepared for them, but still, she knows that for every costumer that enters her shop. So not strange at all.
The world of xxxHolic is Tokyo of modern Japan, even if the shop is in another dimension, people with wishes can see it, aside that differently from Watanuki, Yuuko is able to exit the shop when she needs to do so. Also most of the story happens in the shop, there are few chapters that happen outside, or at least most of the chapters begin in the shop.
In Tsubasa: Reservoir Chronicle , at the beginning she doesn't appear much in the various dimensions, just when the group needs her, the contact her via Mokona Soel, which is normally seen in both the series.
H - means xxxholic manga (english version) and T - means Tsubasa Reservoir Chronicle manga (english version)
1. The first meeting of the group from Tsubasa: Reservoir Chronicle and Yuuko is when the four of them come from various worlds to land in the shop's garden. (H – Vol 1 p.173-176 + Vol 2 p. 6-30 and T – Vol 1 p. 86-116)
2. In T- Vol 1 p. 169, Mokona Soel asks if she can eat the apple. In H- Vol 2 p. 42-43 Mokona Lang chokes it to Yuuko.
3. The group contacts Yuuko, because they need help, the price for help is Fai's wizard staff. (H – Vol 3 p. 8-16, T – Vol 3 p. 81-86)
4. Yuuko sends gifts (Watanuki made them) for Valentine for the group (H – Vol 4 p. 8-9, T – Vol 5 p. 156)
5. Yuuko receives a communication from Mokona Soel, and says that the café Fai is opening has to be named Cat's Eye. ( H – Vol 4 p. 77-81, T- Vol 5-6, couldn't find the exact moment, but we can see that the café is named Cat's Eye and there is point where Mokona says she/he/it has to speak to Yuuko)
6. H– Vol 5 p.4, Yuuko complains she didn't get the White Day gift from the group and that she can't seem to contact them. T – Vol. 8 p.16, the group get a letter from Yuuko where she says she wants her white day gift.
7. Ashura from the Tsubasa: Reservoir Chronicle contacts Yuuko for a wish (T – Vol 9 p.148-152) and then Mokona Soel glups down Ashura and Yasha's swords, while Mokona Lang chokes them out (T – Vol 10 p.80 p.87); during that time Yuuko is in another place to make Ashura's wish come true (H – Vol 6)
From here on, I'll keep listing where in Tsubasa: Reservoir Chronicle she appears, in I'll also try to speculate when in xxxHolic this happened. It's where she begins appearing in the second series more and more.
8. T – Vol 11 p.80-91, Yuuko contacts the group to tell them that she now owns their original clothes, that they forgot in the country of Shara. In xxxHolic this part is probably in the beginning of the volume 7 around page 5, I presume that, because she wears the same clothes as she is seen in the communication.
9. T – Vol 12 p. 161, Sakura contacts Yuuko to give her a dress she made herself. In xxxHolic is probably again in Vol 7 around page 156, she was getting dressed when speaking with Sakura, and she has that dress.
10. T – Vol 13 p. 176, a worried Mokona Soel, contacts Yuuko and tells her about sleep habits. In xxxHolic it probably goes in the Vol 8 around page 113, where she is looking after a sleeping Watanuki.
11. T- Vol 14 p.152, this time is not a contact, but she is shown that she knows that real Shaoran in captivity has woken up. In xxxHolic Vol 9 around page 34, she is dressed the same as in the Tsubasa: Reservoir Chronicle volume.
12. Shaoran Li appears at Yuuko's shop, and he needs to go to the group (H – Vol 9 p. 151-156 and T – Vol 15 p.150-160)
And from now on it's better to work simply on the Tsubasa: Reservoir Chronicle part, because from Volume 16 of the series she appears more in that series than in xxxHolic.
So in the T – Vol 16 p.56, p.60 – 71, is where Yuuko watches the battle between Kamui and real Shaoran, while telling them the story of the clones. During that same volume p. 170-178 and beginning of Vol 17 p. 5-43, Yuuko contacts them to explain who real Shaoran is, and because they need to save Fai's life (Subaru, Kamui and Kurogane have to pay the price for her help.
From the volume 17 the real story of Tsubasa: Reservoir Chronicle begins, the
climax we can say slowly. Aside the fact that the story begins to crossover
so much, bringing it so difficult to arrange it by pages, I would end, writing
down most of the pages from both series.
During the said volume Yuuko begins appearing more and more, giving to the characters advice, which they naturally must pay in different ways.
For those who read both the series, would notice that at some point in xxxHolic, precisely in volume 10, the price that Neko spirit has to pay is to show a well with pure water, that later Watanuki has to bring back in twelve jars. That same water appears in Tsubasa: Reservoir Chronicle Vol 18, when Yuuko using Mokona Lang sends thru Mokona Soel the twelve jars, that are then used for the destroyed city of Tokyo and purified by one of Sakura's feathers that she doesn't want back. The price for this is the egg, that comes doubled to Yuuko's dimension and from one Tanpopo is born, we still don't know what use has the other, but Doumeki still keeps it.
During the T- Vol 18, we finally get some answers, or at least Yuuko is more talkative about the whole ordeal, knowing that the end is near. She gives the name of the man behind the killings and also the group's problems: Fei Wang Reed. Fei Wang's intention was sending them in different dimensions for his plan to work. (by the looks of Yuuko during the conversations, we can say that the time line in xxxHolic is during Watanuki's big injury).
From T – Vol 19, Sakura begins taking decisions for herself, hiding from the group her ideas, she contacts Yuuko without nobody knowing, she even asks Mokona Soel to sleep while she speaks with her. We find soon out, what her wish was, she needed help to be able to go to dream world, and so her body goes in Fai's world, while her soul goes in the dream world, in a way in the second tube that Yuuko has in her shop. Where we find out that Sakura is also the clone of the real Sakura.
Now we have to jump to the T – Vol 22, which is the beginning of the end for Yuuko, and she knows it, and she acts on it, being fully prepared for what is to come and on till volume 28 (last one). While in xxxHolic, she is shown in different lights, mostly like a bossy woman who likes to drink, in Tsubasa: Reservoir Chronicle we mostly see her serious side (at least at the end) and also the tragic nature of her non life. It's in Tsubasa: Reservoir Chronicle that we find out that she has been dead even before both series begun and it's in Tsubasa: Reservoir Chronicle that she gives the explanation of her unnatural life.
At the end we find out that the store, along with the two Mokonas were made especially for the finish, or we can say to write the word end to the Tsubasa: Reservoir Chronicle series. To help the group win and not let Fei Wang resurrect Yuuko.
The truth about Yuuko (at least a part of it) it's found out in the Tsubasa: Reservoir Chronicle last two volumes. As I already spoke about it in her section and the sections dedicated to Clow and Fei Wang. The secret about her existence is the fact that when she died, Clow wished for a moment for her to open her eyes again, and because of his big magical power, it happened, and she become trapped in the time. And then the greed of Fei Wang, who wanted to resurrect her, just to show he was more powerful than Clow.
She vanishes from all the worlds in the T – Vol 27, while Watanuki sees her for the last time in H – Vol 15.
But there is a sequel to xxxHOLiC she could be back at some point, with CLAMP you never know.