The Mother Spider Demon was a member of the Spider Family.

The Mother Spider Demon is a short woman with long white hair, red markings on her face, and large breasts. She wears a long white kimono with large sleeves.Her original appearance was completely different from her current,being small almost child like,having simple black hair and X like marks in her pupils and her breasts being not even visible,hinting that she had to extensively alter her appearance to match Rui's desire for a mother.

Like most demons, she is cruel, unfeeling, and murderous towards humans. Though after much time with him, she developed fear for Rui and the Father Spider Demon who abused her for being weaker than expected, this abuse was severe enough to cause her to fully embrace death instead of having to continue on knowing that both Rui and the Father Demon would continue to abuse her, in her final moments she seemed to have regained some of her feelings and memories as a human and seemed to have been genuinely touched by Tanjiro's kindness that she bid him good luck as she died. It was later revealed that compared to the rest of her "Family" she was technically the youngest demon of the "family" and was still copping with her previous human feelings and memories causing her to cry and grieve many times when remembering them, due to this she became a target of severe reprimand from Rui,due to her inability to be a proper "Mother" to her "Children",who were all technically older than her.

(Source: Kimetsu no Yaiba Fandom)