The younger daughter of the elder from the Tsuwabuki family.
She was destined to fulfill the ritual to keep the seal of the Mount Fuji´s demon safe at the cost of her life, but her father, in an attempt to save her from that destiny, created a younger cloned version of her, so that the clone, Ayumi Tsuwabuki, could take Mayumi´s place in the ceremony. She treated Ayumi in a reckless way because she saw her as a mere object and never stoped to think that she might had feelings, just as a normal human. When Ayumi escaped and ran away with Ren, she went after them with her assistant. She managed to succeed but ones in her house her sister Kureha captured her with the intention of scarifying her along with Ayumi in order to complete her plan of gathering all the energy of the beast under the mount. When Ayano, Ren and Kazuma entered the place were she was imprisoned they saved her and Ayumi and confronted Kureha, but it was to late and the demon awoke. Kureha discovered that all her life she was manipulated by the monster, who suppressed her power to control the earth since she was a baby in order to make her resentful with her family, that saw her as someone useless. Because of this Kureha when berserk and attacked the beast but was killed by him. After a difficult fight Kazuma, Ayano and Ren were able to defeat the demon, but only thanks to Ayumi´s power. Later Ayumi lost her life due to the short life spawn she had as a clone and the amount of energy she had wasted fighting the beast. It was in this moment that Mayumi recovered her consiusnes and realised the mistake she had commited treating Ayumi in the reckless way she did, and apologised.