Medusa's sister, and the eldest of the three Gorgon sisters. She is also known as "The Heretic Witch". She is "mother of all Demon Weapons" and the leader of "Arachnophobia", a massive organization that uses Baba Yaga castle as its main base of operations. She was defeated by Shinigami 800 years prior, which causes her to seal her soul into a golem guarded by Giriko until she is able to revive herself. She does not get along with her sister, and prefers to manipulate the enemy from afar rather than face them in direct combat. She is able to immobilize enemies with spider silk, imprison someone within their own mind and force them to face their own fears, and use a number of spiders for reconnaissance.
In the manga, she attempts to turn herself into an incarnation of insanity so that she can absorb Asura. Her soul is defeated by Maka and her body is taken by Medusa, though residual aspects of Arachne are still present.
In the anime, she convinces Asura to join her in order to spread insanity, though he eventually kills her and eats her soul.