先代博麗の巫女 (Sendai Hakurei no Miko, "The previous generation's Hakurei shrine maiden")
Depicted as Reimu's predecessor. She was apparently not created with a proper given(first) name, so her name may vary. Usually portrayed as a powerful physical fighter.
She wears a customized miko uniform consisting of a one-piece skintight leotard rather than a sarashi (frequently resulting in sideboob), sturdy combat boots, and sometimes hakama pants rather than a skirt.
Also, unlike Reimu, she does not wear a ribbon on the back of her head, although in some depictions, she wears one to tie her long hair together near the end. Some artists depict her as muscular and/or covered in scars.
She is usually depicted as Reimu's mother, though as a fan character her relation to Reimu on the Hakurei family tree can change with each depiction.
A very popular fanwork featuring this character is Osana Reimu and its sequel Reireimu, a story about Sendai Miko, a young Reimu, and an adult Rumia. This version looks almost identical to Reimu herself, only taller and more endowed, and oftentimes wearing a mask of some sort.