Mii

ミィ

By performing a highly bouncy and suggestive dance, Natsumi Rokudo can transform into Mii, the voluptuous Flower Spirit and Fertility Goddess of Myauginiya. Through dancing, she can also fly, grow to a gigantic size, and, naturally, look incredibly ridiculous.
Her enormous breasts are said to represent and hold Earth's life-giving energy. When necessary, Mii can also enlarge her entire body to giant proportions, which causes her already massive breasts to grow larger in proportion to her overall size.
According to Ongo, Mii was rumored to have been Ahem's wife, but she left him because she could not stand him. This presumably occurred before the evil within Ongo took hold and went on a rampage. Ahem was unable to stop him alone, so Mii gathered the Earth's energy to immobilize Ongo while Ahem sealed him in a statue. This statue would be discovered many years later by the archaeologist Fuyuhiko Rokudo, who gave it to his daughter Natsumi as a gift. After Natsumi unwittingly breaks the seal on Ongo's statue, Ahem gives her the remainder of Mii's power, allowing her to transform into the goddess after performing a sacred (perverted) dance.
Since Natsumi is usually the one transforming into her and thus in control, Mii's actual personality remains unknown. The flashback in episode 3 does show that she cared enough for nature to sacrifice herself to stop Evil Ongo's rampage long before the series began. She still seems to retain some presence, as Natsumi heard her voice during the gigantification dance.