Eternal Vows

The heroine is an ordinary student. One day, she finds herself thrown back in time to an old era, about 500 years before, known as the “Warring States” period. She meets ninjas who are fighting and risking their lives in major battles.

There she meets Uesugi Masatora (later known as Uesugi Kenshin).
Masatora is interested by the fact that she comes from the future, and so he invites her to the castle as an "Envoy of Bishamonten". There she begins to be protected by a group of ninjas called "Nokisaru".