The Mysterious Officer (謎の将校?) is a member of the German Third Reich during the Imperial Capital Holy Grail Strange Story of Fate/KOHA-ACE and Fate/type Redline. He is given command of a test Heroic Spirit Soldier, Cú Chulainn in Fate/KOHA-ACE and Lancer in Fate/type Redline.
The Mysterious Officer appeared to be a middle-aged man of average build, with short, light coloured hair. He wore a standard military uniform with a fur lined coat over it.
The Mysterious Officer was a rather arrogant and callous individual, taking the power of his servant for granted, he ordered Lancer to kill both Tsukumo and Kanata, despite one of them not being involved in the war at that point. When his servant was killed, he attacked Archer from behind with his sidearm, declaring that 'you monkeys' can act strong, but in the end it would be the Third Reich to rule the world, showing that he is both perhaps cowardly and racist.
On a train, the officer and his Lancer threaten Tsukumo Fujimiya. When Kanata Akagi mysteriously materializes on the train, the officer casually tells his Lancer to kill them both. Kanata inadvertently summons Saber who quickly kills the Lancer. Later, while Saber is fighting Archer, the officer tries to shoot Archer in the back twice, but Archer blocks both bullets. The officer furiously rants that they are just monkeys and that he and the Third Reich will win the Holy Grail War and claim the world. Annoyed, Archer shoots him in the head, destroying it and killing him instantly. After Archer destroys the train and retreats, Kanata finds his corpse in the wreckage. Later, Imperial Japanese Army officers return his corpse to the Third Reich with an apology for the trouble Archer caused them.
The Mysterious Officer did not seem to possess any skills in magic, instead relying on what appeared to be a standard sidearm and his servant in combat. It is likely he only possessed vague knowledge of mystics by his association with the Third Reich as he recklessly attacked Archer from behind with nothing more than a pistol, an approach even low rank magus would consider suicide.