Mii

Through a very bouncy and suggestive dance, Natsumi Rokudo can transform into the voluptuous Mii, the Flower Spirit and Fertility Goddess of Myauginiya. By dancing, she can also fly, become gigantic and, of course, look amazingly ridiculous.

Her massive breasts are said to represent and contain Earth's life-giving energy. Mii can also enlarge her whole body to giant size when needed, which causes her already humongous breasts to grow larger proportionally as she does in size.

According to Ongo, Mii was rumored to be the wife of Ahem, but she left him due to not being able to stand him. This was presumably before the evil inside Ongo took hold and went on a rampage. Ahem was unable to stop him on his own, so Mii gathered the energy of the Earth to immobilize Ongo while Ahem sealed Ongo in a statue, which would be found many years later by archaeologist Fuyuhiko Rokudo, who would give it to his daughter Natsumi as a gift. After Natsumi unwittingly breaks the seal on Ongo's statue, Ahem gives the remainder of Mii's power to her, allowing her to transform into the goddess after performing a sacred (perverted) dance.

Because Natsumi is usually the one transforming into her and thus in control, Mii's actual personality is unknown. The flashback in episode 3 does show that she cared for nature enough to sacrifice herself to stop Evil Ongo's rampage long prior to the series. She still seems to have some presence, as Natsumi heard her voice in the gigantification dance.