As a boy that always felt out of place, not feeling comfortable with his body and with a complex due to his small height and the unusual spelling of his name (that reads as "Virgin" in Japanese), his didn't feel comfortable with her body until the special strain of the Armed Virus infected and turned her into a beautiful transgirl. Shortly after she met Mirei (a fellow transgirl) in the Mermaid island. Now they are always together with a deep love blossoming between them.

An innocent, naïve and childish girl, she is often made fun of because her surname is written with the same kanji for "virgin" (処女 shojo). She is an Extar whose transformation is a large curved sword, though she later gains the power to transform into a double-bladed spear through the Valkyrie Effect.

Unknown to herself, her power level is far greater than Mermaid Island's other inhabitants. As such, and with her compatibility with Mirei, she is able to unleash the Valkyrie Effect, the ultimate manifestation of the powers of the A-Virus which can manipulate - or even cancel - the effects in other carriers.

She liked Guru-Neko since childhood. Previously she was a second-year student at Tokyo Sakuragaoka High school.

The Armed Virus is a strange disease that only affects women and enables them to turn into weapons when sexually aroused by another carrier. The ones that turn into weapons are known as Extars, while the ones who can trigger the transformation and wield the resultant weapons are known as Liberators. Mamori is an extar and Mirei is her liberator.

Lady J and Hasumi Rain, a powerful couple of extars known as Lady Lady are one of Mirei's most trusted and valuable allies.

Mamori's dream is to find a cure for the armed virus and return home alongside Mirei and their friends to live happily and proudly as transgirls, able to indulge in all of their sexual urges.