Watanuki is a high school student whose life has been a rather difficult one—not only have both his parents died, forcing him to live alone, but he is plagued daily by spirits that only he can see (both the ability to see them and the cause of their attacks are evidently heritable traits—apparently Watanuki's blood smells very delicious to spirits).
One day, Watanuki enters into an agreement with Yuuko to work for her in exchange for the fulfillment of his wish–to never see spirits again. This marks the beginning of Watanuki's very involved part-time job, a fated meeting with a certain quartet from Tsubasa: Reservoir Chronicle, and so many "chance" happenings which occur not out of coincidence, but "hitsuzen".
Watanuki is known for a having both a volatile and caring nature to him. Depending upon the person he talks to, his emotions tend to be overly exaggerated. With Himawari he throws himself into the air accompanied by hearts and a "Himawari-chaaaaan!!" squeal. With Domeki, he almost always expresses irritation with various facial expressions, screaming, and arm motions that reminds Himawari of an angry cat. With the exception of these two and Yūko, Watanuki tends to address everyone around him with formality and kindness.
One of Watanuki's greater problems, especially early on in the manga, is his lack of self-worth. He found absolutely no value in himself and would often use himself to aid another, even if his life were at stake.
An example of this is when he willingly gives up his right eye to the Spider's Grudge to save Domeki's, without considering how much his decision might hurt Domeki. He's forced to face up to this problem when trying to save the Zashiki-Warashi from the Joro-Gumo, when the spider matron harshly chastises him for his lack of consideration for both his own self and those who cared about him. Faced with the facts of how much he's hurt those around him, Watanuki begins to see some value in himself. He will still often use himself to save others, but now he is mindful of just how much his actions hurt other people.
Life for Watanuki changes significantly once he starts working as Yuuko's assistant (rather, her very indignant indentured servant) who not only cooks (wonderfully), cleans (diligently), and complains (constantly), but finds himself performing very odd jobs and learning more about the occult.