...who she could be...
Yuuko is such a mysterious character that sometimes I try to find out what is going on and search for the explanation about who she is, who she was, especially because she alone says that Yuuko Ichihara is a false name, so maybe we met her in some earlier CLAMP works and we never knew. You never know, especially with them. This page will be updated every time I get an idea, and please bear in mind: THIS ARE SIMPLY MY IDEAS NOT FACTS.
1st speculation: Tsukiko from Gouhou Drug
The first time I thought she was Rikuou’s Tsukiko. I came to this conclusion because she is said has vanished and Kakkei never was able to “see” her, so it means she had some powerful magic.
But then my idea got killed in xxxHolic Vol 1, chapter 5. She can’t be Tsukiko because Watanuki sees a female spirit attached to Rikuou when he goes to the drugstore, so we know her spirit is attached to the boy and can’t be Yuuko, but hey it was worth a try.