From Gamerz-Edge Anime:

Kekkaishi follows the story of Yoshimori Sumimura and Tokine Yukimura two separate Kekkaishi (an elite breed of Yakashi or demon hunters) who have been guarding the sacred grounds of the Karasumori castle for over 500 years. And to make things interesting both Yoshimori and Tokine come from different clans of Kekkaishi that have been fighting each other for the title of the true Kekkaishi since the original founders death. While the young Tokine is very skilled at her job, Yoshimori is a little on the slow side which only compounds their relationship as Tokine repeatedly has to come to the rescue of the young Yoshimori.

As the story develops Yoshimori makes a pact that he will never let Tokine be injured after one faithful night when Tokine almost died protecting him. Yoshimori with his new mission has to struggle hard for the acceptance of Tokine as their childhood friendship crumbles around them after long nights in the field. As with any good Manga, there’s always something else thrown in to spice up the world a little bit, and Kekkaishi is no different as a ghost of an ex pastry chef, Yoshimori’s stubborn old grandfather, and the ever problematic school life begin to interfere with Yoshimori’s life as a Kekkaishi.