Mirei Shikishima and Mamori Tokonome are a lesbian couple afflicted with the dreadful Armed Virus that causes Mamori to turn into a sword whenever she has an . Due to this power, many are interested in Mamori but Mirei has made her life goal to protect Mamori and return her home whatever the cost.

Infected with the Armed Virus, Mirei was raised as a soldier. When she failed to meet the objectives of her trainers she was sent to Mermaid Island, the island where armed virus carriers are quarantined for everyone's safety. Mirei has only known death and destruction until she met Mamori Tokonome on Mermaid Island. Since then she has made her life goal to protect Mamori and help her with everything she needs. She's a proud lesbian and is deeply in love with Mamori.

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. Mirei is a liberator and Mamori is her extar.

Mirei is a former assassin of Feste, where was given the name C7. Because Mirei was refusing to fight, the organization decided to dispose of her. A doctor helped her and sent her back into the world, reporting that she was killed during the war a year ago. She was unable to live a normal life and was sent to Mermaid when she found a partner and a reason to live in Tokonome Mamori.

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.

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 as normal girls.