I'm sure about a question. It must exist a reason if Clamp created XXXHOlic. TRC and Holic is being published at the same time, but both follows differents ways. TRC has a lot of action and few explanation, while, in the other hand, XXXHolic, with Yuuko, has a lot of things to tell and less action than TRC. Connections isn't few.Yuuko knew Clow, so she also knew SAkura & Syaoran. FWR is a "big" enemy to her. Yuuko's behaviour is really calmed, so she always knows what to do or if her past actions were good. I sense that Yuuko was created by Clamp 'cause she'll give an explanation at the end of both stories.
also, both Mokonas were created by Yuuko & Clow. White for Syaoran, black for Watanuki. They're guards, because "manjuus" have saved a lot of times the kids.
Yuuko, in the manga, mentioned that the White Mokona represents "Power", and the Black Mokona represents "Seal". Which would make sense since Watanuki needs to shield himself from the strange things that happen to him, while Syaoran and co. need as much power as they would need to face whatever danger comes in their way.
But wait... wouldn't Watanuki need power to fight the weird things, too?
Remember, he has power already, but he needs to learn to control it, not the other way around, plus he has a wish to get rid of his power. Syaoran and co., in exchange for the ability to travel across the dimensions, gave up whatever weapons they might have in the first place, and Syaoran never had any special powers to begin with himself.