Manga by author Ueda Rinko - Status ongoing

Ryo thinks she is a normal girl living a normal highschool life in modern day Tokyo. But is she? On a school trip, a mysterious man attacks her and claims that he is Benkei, the legendary warrior monk who collected 999 of his opponents' swords in battle during the Heinan era. And he also claims that she is Ushiwakamura, the 1000th opponent, that according to folklore, beat Benkei and became his master. But...Ushiwakamura was a man! And Benkei and Ushiwakamura existed in 13th century feudal Japan!

From Decadence: Spring of 1921 in the Taisho era, Ichijo Maria had lost her parents and entrusted herself to a friend's family. She leaves the house when the father became physical with her. Maria wanders the town and collapses in front of her old house. She is saved by a young doctor called Toudou Sei. There, she learns the house is currently being used as a dorm by the doctors of Imperial Maria Hospital. Remembering her fathers words, "I was tricked by Toudou!" she works there as a maid under a fake alias in order to reach the truth...