Great spiritual power is required to see spirits, but it takes even greater power to bind spirits to your will.  Natsume’s grandmother, Reiko, had enough power to beat spirits in games and bind their souls to their names which she collected. The names she collected were put in a book she called her “Book of Friends”. Perhaps she referred to the spirits as her friends because she didn’t have any human friends.

Regardless of who she thought of the spirits, they now want their names back. However, Reiko is long dead and it is now up to Natsume to return the names taken out of loneliness with kindness.