Seiga is a "wicked" hermit and the superior of Miyako Yoshika. She has the ability to pass through walls.
Her ability does not function by making her intangible; instead she uses her hairpin to physically cut holes in walls, which disappear after a while. The wall need not be vertical, and she can use her ability on the ground to create tunnels. Walls made of soft materials appear to be more difficult for her to pass through than hard ones; she can use her ability on liquid walls, but the hole will be destroyed instantly.
(Source: Touhou Wiki)
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