This is a "shikigami" that was placed in Watanabe's hair. There are few people who have seen it, but according to one theory, it is six giant weasels.
The spiritual power is amplified by the tattoo spell and flows into her hair, and from the braided hair into six "shikifuda"(Tags made of Japanese paper). It is said that the spiritual power is stored until just before it is used, and then torn off to be used.