Quote from Keel
Also is it possible for Watanuki to be the combined in carnation of Clow Reeds Coul. As a majician he doesn't hold much talent but the darkness is atracted to him, he holds close ties with Syaoran, and the girl he likes bears close resemblance to his wife in clow country. Any thoughts?
Read Tsubasa chapter 201, its starting to clear up who Watanuki is and his connection to Syaoran
Otherwise read spoiler below...
Spoiler (highlight to view)
It seems that Watanuki is Syaoran (a bit like ying/yang), when Syaoran gave up his relationships, time etc to grant some wishes Watanuki was born as Syaoran's parent's child in Syaoran's place, and so as the evil guy says Watanuki is Syaoran's self