The Mother Spider Demon was a relatively short and voluptuous woman in her 30's (or its demon equivalent), 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 kimono — 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 nail polish on her toenails; the lower front of the kimono is open, exposing her long legs.
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, Mother Spider Demon is cruel, unfeeling, and murderous towards humans. After much time with Rui, she developed fear for him 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 therefore helps him by warning him that a Twelve Demon Moon was on the mountain and 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 (apparently, the equivalent of a pre-teen girl in demon terms) and was still coping with her previous human feelings and memories, which made 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 Mother Spider Demon's past as a human, but she was loved and cherished by someone, a young female person with a maroon haori and black hair styled in a low cut bang with a ponytail. In her final moments, she seems to have remembered some parts of her life as a human child such as remembering being loved and cherished by this person.
After she became a Demon and lost her loved one (the implication is that they somehow died in a rather bloody manner), she took up residence at Natagumo Mountain. Later, Rui came to Natagumo Mountain, convinced her to join him, and then 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 child demon, she was unfit for the role and was met with chastise and abuse from both Rui and the Spider Demon (Father).
Rui assigns his "Mother" the role to protect the Natagumo mountain and her 'family' from the demon slayer corps. She sits on a huge rock in the middle of a forest clearing and, from there, she uses her spiders' 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, Mother's threads become harder and her "dolls" (still living demon slayers) get stronger. She then gets confronted by Rui, who asks her if she can win and tells her that 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, which causes the Mother to freak out.
Now in a state of panic, Mother Spider Demon resorts to using the living but most grievously 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 through immense pain due to the physical strain. She then manipulates them, and especially a young woman named Ozaki, with her threads into attacking the duo; as she does so, Ozaki tearfully shouts all the info about her and the others' situation she can recall to Tanjiro, while also begging him to leave and get help.
Tanjiro, unwilling to kill Ozaki and 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 Slayers' necks (including the unfortunate Ozaki) and resorts to her strongest puppet as she enters into an even higher state of panic In the midst of a state of borderline hysteria, she releases and reveals her strongest puppet that is revealed to be a headless corpse of a Fellow Demon which she somehow collected, that she then launches into battle against Inosuke 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", Mother Spider Demon 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, then 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, Mother Spider Demon holds her arms up awaiting death from Tanjiro's coming attack. Tanjiro, realizing that she no longer has any intention of fighting back, finishes her with the Water Breathing: Fifth Form - Blessed Rain after the Drought instead of the First Form: Water Surface Slash 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 merciful 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 Kizuki is in the mountain and bids him good luck as she dies.
Shapeshifting: The Mother Spider Demon has shown advanced skills in the shape shifting, 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.
Spider Physiology: Like the rest of her "family", the Mother Spider Demon has spider-like traits and abilities.
- String Manipulation: Like the other members of her "Family", the Mother Spider Demon's main form of attack is focused on spider threads/strings created from her flesh and cells. She materializes these strings from the tips of her fingers and can extend the range of the strings to cover an entire mountain.
- Puppetry: The Mother Spider Demon uses her strings in a unique manner by utilizing it to manipulate corpses like puppets. She does this by attaching her strings 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 strings 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. The closer the strings connected to her are, the more power she can exert over the puppets she manipulates.
Spider Manipulation: The Mother Spider Demon can create and command small white spiders with red dot like design similar to the physical traits of herself and her "family" to stealthily attach her threads to her intended victims without them noticing. These spiders also have similar traits to demons, as they burn up upon getting stabbed/sliced by a Nichirin Sword.
- Mother Spider Demon was ranked in 34th place as of the first character popularity poll with 32 votes.
- (To herself about Tanjiro Kamado) "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."