This is a bird skull that 7 wears. It serves as a helmet. Two long blue feathers are attached to the back that are possibly used to cover the scar that crosses out her number. She loses it near the end of the film when the fabrication machine sets off a bunch of grenades on her, 9, 1, and 3 & 4. It is possible that 7 discovered the bird skull when (or if) she was imprisoned in the birdcage that 2 is seen inside when captured by the Cat Beast in the film. The bird that the skull belongs to is perhaps a canary, and may have been owned by the Chancellor.