Kekkaishi Episode 1 – “Scars”

Nine-year-old Yoshimori Sumimura is a kekkaishi (barrier magic user) and reluctant heir to his family. Together with his neighbor Tokine Yukimura, age 11, he is responsible for defending their school from ayakashi (demons) attracted to the spiritual energies of its site. When they are found chatting, Yoshimori’s grandfather and Tokine’s grandmother chastise them for talking with someone from their hated rival family. That evening, Yoshimori traps an ayakashi but is tricked into not dispelling it until it grows too strong for him, and Tokine is injured while saving him. As she recovers, Yoshimori vows to never let anyone else get hurt by ayakashi, and five years later continues defending their school with his full strength. (source)