Mamori is an innocent and naive sixteen-year-old girl who, at a young age, overheard her parents talking about her illness. However, she didn't think much of it until she was abducted and sent to Mermaid island, where she became Mirei's partner. She is often made fun of because her surname is written with the same kanji for "virgin." 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's other inhabitants; as such, and with her compatibility with Mirei, she is able to unleash the Valkyrie Effect, the ultimate manifestation of the Armed Virus' powers which can manipulate - or even cancel - its effects in other carriers.