Summary
In the world ofDemon Slayer, not all demons are evil and some aid the Demon Slayer corps with the ongoing battle against Muzan. Lady Tamayo is one of the demons supporting the demon slayer corps. A doctor and friend of Tanjiro, Tamayo was born during the Oei era of Japan and encountered Muzan when she was very young and sick, seeking a cure for her illness.
Muzanimmediately turned her into a demon, causing her to run amok and kill her family and other innocent humans. However, Tamayo was one of those demons who still felt their human emotions and deeply regretted what she had done.

The Demon Who Switched Sides
Lady Tamayo Aided the Demon Slayers in the Fight Against Muzan
Tamayo harbored deep hatred towards Muzan andsecretly wanted to avengeher family by killing him, but she soon realized that she was no match for his powers, as he was a thousand times stronger than she was. Because of this, she served him by his side and became his servant. One day, things took a turn as Muzan and Tamayo were walking through an area known as the bamboo forest and came across the legendary swordsman, Yoriichi Tsugikuni. Up until this point, Muzan was undefeated, and the many attempts to take his life had failed by powerful swordsmen. Thinking that he could defeat Yoriichi,Muzan tried to kill himbut was shocked when he realized that Yoriichi was able to evade his attack. Before Muzan could react, Yoriichi had already executed a move that sliced Muzan into pieces, causing him to flee from the scene.
I always ask them beforehand if they wish to live on even as demons.

It was at this point Tamayo realized that there was a force powerful enough to take down Muzan. Yoriichi spares Tamayo and lets her escape. He believed that she would be an important part in defeating Muzan and believed in her plan to defeat the demon king. She was now free from Muzan and was able to act freely, but shechose to dedicate her lifeto studying demon biology and looking for ways to kill Muzan. Initially, Tamayo survives by consuming human corpses and animals, but as the years go by and after 200 years of research in medicine, she is able to alter her body, so she is able to live her life without killing humans. Tamayo also later comes across a young boy named Yushiro who is sick and alone. Tamayo heals him by using the same medicine she used on herself on him, turning him into a demon. In return,the boy stayed by her sideand became her assistant.
An Expert in Demon Physiology
Lady Tamayo Was Extremely Knowledgeable About Demons
Thanks to her dedication to learning everything she can about demons and medicine, Tamayo is extremely knowledgeable. Her understanding of the anatomy of demons plays a vital role in the series. She also learns how to convert demons back into humans. Even though she may not be physically as strong as her demon peers,Tamayo makes up for itwith her vast knowledge of medicine as she was able to alter her body so that she could survive on a small quantity of human blood and paid donors for their blood instead of killing. She was also able to develop a drug that could weaken Muzan and this played an important role in the final battle against him.
Her dedication to eradicating the world of Muzan was limitless, and she stopped at nothing until she was sure Muzan was dead. Tamayo also sacrifices herself in the final battle, but her legacy endures as her work paved the way to ensure a world free from Muzan’s reign. A compassionate and intelligent demon,Tamayo is forever rememberedand revered as a major supporting character who shook the demon world to the ground.

Demon Slayer
Based on Koyoharu Gotouge’s shonen manga, Demon Slayer: Kimetsu no Yaiba is an action anime set in Japan during the early 20th century. Seeking revenge for the murder of his family while also looking to save his sister, Tanjiro joins the Demon Slayer Corps as they try to eradicate Demons led by Muzan.