- Medusa (メデューサMedyūsa?)
- Voiced by: Hōko Kuwashima (Japanese), Luci Christian (English)
- The first major antagonist of the series, a snake-like witch who researches the effects of black blood, hoping to use it to control Asura, the first kishin, and reshape the world in her own image. To carry out her experiments, she infiltrates DWMA as a kindly school nurse. She is sinister, scheming, and incredibly cunning, allowing her to amass a small but effective entourage to carry out her plans. Her body contains 1,000 magical snakes which can be used as weapons called "Vector Arrows," as well as to place them inside the bodies of others to communicate with them telepathically or to kill them from the inside. She is killed in battle with Franken Stein shortly after her plan to revive Asura succeeds, though her soul persists and possesses a young girl named Rachel. She later forms a truce with DWMA with the magic tool "Brew." In the manga, she assists DWMA in defeating her sister, Arachne, only to possess her sister's body and continue her experiments on the black blood in Crona, which culminates with Crona eventually killing her. In the anime, she manipulates Stein through his increasing madness, but is defeated by Maka.
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