Xing Hou is the mysterious assistant to Fei Wang Reed (or Fei Wong Reed). Little is known about her except the fact she is partly a "failed creation." However, as the story progresses her character has more depth. In Volume 15 of the manga she helps the real Syaoran go to Yūko after he awakened (Volume 09 of ×××HOLiC), but stated she can only do it once. Her reason to him was "If the dream does not end."

Spoiler

She is killed by Fei Wang shortly afterwards, using the same blade used to kill Kurogane's mother. Her helping Syaoran was inevitable, as Fei Wang Reed states "Considering your origins, I suppose it couldn't have been helped". In Chapter 178 she made an appearance in a flashback of her and Fei while he was explaining the clone soul of Sakura, along with other girls who are also cross over from other Clamp works.

Sources: Wikipedia; CCSWikia