A few weeks before the war began, Major Reiter and her superior went to Major Magatsu to form a new alliance between the Third Reich and Imperial Japan for the Holy Grail War. In exchange for a hiding place within the imperial capital, she asked that Major Magatsu summon a Berserker-Class Servant, and that they will provide him with the IV-Type Heroic Spirit Restraining Device (IV型英霊拘束具?) so that he can control his Servant.
She has purple shoulder length hair. She has a mole under the left side of her lips. She wears a black uniform.
She seems to be a reasonable, calm and intelligent woman. Major Reiter is not reckless like her Servant, and she uses her first Command Seal to force him to retreat from battle since she needs still him to be useful. She is good at reading people and observing them (such as when she is having a conversation with Major Magatsu). She also tolerates Assassin's tantrums when he yells at her for not finishing off with Saber.
She sends Assassin to scout out the Master of Saber, allowing him to cut through their allies in the Imperial Army because their alliance is only temporary. When Assassin starts torturing Tsukumo Fujimiya, she asks him to stop over their telepathic link, but he refuses and says she does not give him orders. When her Servant is on the brink of being defeated by Saber, she uses her first Command Seal to make Assassin retreat since she still needs him to do his job.
After her conversation with Major Magatsu, she secretly smoking a cigarette and drinking a glass of alcohol. She notices the cigars Major Magatsu has didn't have any sign of magecraft and wonders if he was really trying to gain her trust. She calls her Servant, but Assassin stole a bottle of alcohol from her and tells Reiter not to call him by his class. She tells him that how he is refers as a Servant and asks if he is still mad. When Assassin take out his angry for not finishing off Saber, she explains he was suppose to kill Saber's Master, not the Servant and asks him to focus on his orders. She tells Assassin if he is still so upset, he could cut off her head with his sword as long as he doesn't mind go down a useless fool who can't follow orders. As she leaves, she tells Assassin to keep the bottle of alcohol, get some rest, and await for his next orders.