I haven't read this in a while. But didn't Yuuko die? so how can it be her?

I don't know whether this is true or not., but., Yuuko lives in Watanuki's dream in early chapters..

she is dead and she never come back remenber that thsi isjust some extra chapter because in the end of the manga holic 100 years pass and yuko never came back

Um., I mean in the dream when Watanuki collected the things together with Himawari and Doumeki for Shaoran in this manga.. Although she is only created by Watanuki in his dream..

Well Yuuko is the dimensional witch, and the way Tsubasa left the series was that Yuuko and the clones were in together in a real outside of the multiverse of dimensions in a void. That they could be reached only under certain conditions one of which being that Sakura didn't travel with Mokana and company. That as yuuko and the clones were artificial lives whose lives were linked to many dimensions all at once there was a slim chance of them being able to return to the realms they were created in.