A mysterious girl of whom initially nothing is known, Emi is the initial Factor (pilot) of Linebarrel, and indirectly the one responsible of Kouichi's death. Of his same age, she appears as very polite and calm, though very strict and demanding if the situation arises, although sometimes quite clueless about things that should be perceived as obvious, such as asking what a Christmas party means. She is later revealed to be the younger sister of Katou Hisataka, and thus another traveler from the "other dimension" that spawned the Machinas themselves. Finally, it was she who introduced the special, black Mode-B function of Linebarrel. While her roles in the franchise are pretty much the same, in the anime she appears much more strict and unforgiving; also, during the end of the animated series she is briefly killed, though promptly revived thanks to Linebarrel's own sacrifice.
Emi is the first pilot of Linebarrel, and thus she initially shared control of the Machina with Kouichi Hayase, until the latter took complete control over it. Both in the manga and in the anime she was the first to use Linebarrel's Mode-B in actual combat.