Also called "White" or "Shiro" ("白"). Literally means "White Weaver".
A major character revealed part way into Kumo Desu Ga Nani Ka? She has very long white hair which come down between the eyes and is partly tied in a braid, red eyes that have extra pupils (which she canonically keeps closed to prevent people from seeing), and typically wears a red choker with a flower on the side, a white dress that has very wide sleeves and a side slit, and white boots. She wields a scythe as her weapon.
Remember when tagging that Shiarori's identity is a spoiler, and any posts that are give a hint to her identity should be tagged as such. Do not tag Kumoko unless both characters are actually present in the image.
See also: Her page on the Kumo Desu Ga Wiki