The Mother Spider Demon was a relatively short and voluptuous woman, similar in appearance to her "son," Rui, having the same milky-white skin, teal eyelashes and nails, and circular, red facial markings. Additionally, she had bushy eyebrows and long, white hair parted down the middle of her face which was tied with teal orbs into two, thick strands.
The Mother Spider Demon wore a lengthy, layered kimino — maroon on the inside layer and white on the outside layer — that exposed part of her chest and was tied with a patterned obi. She also donned a beaded necklace composed of teal and purple orbs and decorative anklets. She also goes barefoot and has black toenail polish.
However, it was revealed that this was not the Demon's true appearance as her body was altered to look like Rui's. Her original appearance was completely different from her transformed one, with a much smaller, childlike body compared to her new, more voluptuous figure. Her skin was marked with two maroon lines on each of her cheeks and her eyes were rounder with yellow irises and x-shaped pupils. She also wore her short, black hair parted to the side with bangs framing her face and two, low pigtails.
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 coping 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.
Not much is known about her past as a human, but she was loved and cherished by someone. After she became a demon, she took up residence at Natagumo Mountain. Later, Rui came to Natagumo Mountain, and forced her and her, soon to be, family to transform themselves to look like him and become his fake family. She herself took the role as the mother of this family, but because of her being a very young demon, she was unfit for the role and met with chastise and abuse from Rui and the Father Spider Demon.
Rui assigns her the role to protect the Natagumo mountain and her 'family' from the demon slayer corps. She uses her threads to take control of the demon slayers that enter the territory and make them her own dolls. She uses some of the demon slayers she took under her control to kill the others, having fun playing with them.
Tanjiro and his friends enter the mountain in order to defeat her and her family. They are greeted by her dolls which are dead demon slayers. However, her dolls are easily defeated and Inosuke detects her location.
As they get closer to her, her threads become harder and her dolls get stronger. She then gets confronted by Rui, who asks her if they can win and tells her she is taking so long. She panics because despite the supposed "Mother-Son" relationship between them, she knows full well the massive power gap between herself and Rui. Rui then threatens that he will inform the Father Spider Demon about the situation if she does not kill the remaining Demon Slayers quickly. Now in a state of panic, she resorts to using the most greviusly injured Demon Slayers in order to fight to inflict psychological damage to the duo and she also begins to push the physical power of the controlled Demon Slayers to the maximum, putting them on immense pain due to the physical strain. She then manipulates them with her threads into attacking the duo.
Tanjiro, unwilling to kill his comrades, figures out how he can immobilize them without killing them. Using her threads, Tanjiro picks and throws the controlled Demon Slayers up toward the trees where the the threads controlling them are tangled into the tree branches, stopping her from moving them any further. Deeming them to be all useless to her, The Mother Spider Demon snaps all of the captured Demon Hunters' Necks and resorts to her strongest puppet as she enters into an even higher state of panic In the midst of of which she finally reached a state of borderline hysteria she releases and reveals her strongest puppet that is revealed to be a headless corpse of a Fellow Demon that she somehow collected, that she then launches into battle against Inoske and Tanjiro. When the duo see it they are amazed that it does not have a neck. They start attacking it but their attacks are ineffective against it. So they use a combination attack together that manages defeat it. Straight after that, Inosuke throws Tanjiro up to the sky in the direction of the Mother Spider Demon's location. As she fears defeat and punishment from her "Family", she notices Tanjiro diving down towards her from above. Trying to figure out her next move in Order to survive, she realizes that if she simply dies she will finally be at peace and no longer have to suffer the wrath and ridicule of her "Family".
Having already given up any form of resistance and willpower to continue fighting, she holds her arms up awaiting death from Tanjiro's coming attack. Tanjiro realizing that she no longer has any intention to fight back, finishes her with his Fifth Style: The Merciful Rain of a Dry Day instead of the First Style as originally intended. He instead opted to use a "sword strike of kindness" that beheads the target with little to no pain which is used when the enemy surrenders him/herself willingly to grant them a mercifull death. After she is beheaded, the Mother Spider Demon is shocked at how peaceful and painless her death is and is even further surprised and touched by Tanjiro's eyes, which has no malice, hate or contempt. Instead she sees nothing but pure kindness and mercy. As she fades away she remembers all the painful memories and unkind stares her "Family" gives her and vaguely recalls her past as a human, remembering being loved and cherished by someone and as she slowly disintegrates into black smoke weakly wonders where that person is now. In return for Tanjiro's kindness, she weakly warns him that one of the Twelve Demon Moons is in the mountain and bids him good luck as she dies.
Abilities and Powers
Shapeshifting - The Mother Spider Demon has shown advanced skills in the shapeshifting, as she managed to change and alter her skin color, hair length and color, facial structure, demon marks, eye color and physical characteristics to match and resembles Rui's own physical features and maintain these changes for extended periods of time. However, most likely due to her young age as a demon, she would sometimes accidentally revert back into her true form much to Rui's annoyance making him severely punish her.
- Threads - Like the other members of her "Family", the Mother Spider Demon's main form of attack is focused on spider threads by attaching her threads to her intended victims she can successfully turn them into virtual puppets that she then uses to combat her opponents from a safe distance. She can even use these threads to somehow bring out the full physical power of her controlled puppet to its maximum, albeit at the cost of putting her puppets through severe physical pain due to it.This ability also allows her to control the corpses of Demons, as she somehow acquired and preserved the dead body of a Demon and prevented it from disintegrating to black smoke for use in combat.
- Spiders - The Mother Spider Demon can command small white spiders with red dot like design similar to the physical traits of herself and her "family" (as such they may be detachments from her own body) to stealthily attach her threads to her intended victims without them noticing.
- Mother Spider Demon was ranked in 34th place as of the first character popularity poll with 32 votes.
- (To herself) "Those eyes.... Such kind eyes... Back when I was human, I think someone used to look at me with kind eyes. I wonder who that was. I can't remember. It was someone who always treasured me. I wonder how that person is doing now."