Kazuki's sister and first year student at Kazuki's school. Ditzy and constantly cheerful, Mahiro seems to lack common sense and sees nothing wrong with things like letting her brother borrow her school gym clothes. Her quirks and personality are so similar to her brother's that no one ever doubts they're related despite the fact that they don't resemble each other. She secretly wishes that Tokiko and Kazuki will get married so that Tokiko will be her new sister.
Mahiro discovers that her brother is an alchemist warrior when the LXE attack the school in force and understands that Kazuki must sometimes go away on missions, but decides not to probe him for information about what he does as an alchemist warrior.