Most of them are

protagonists (usually women) who were originally modern people

Because of some accidents, Sudden crossing into ancient times

can be an era that existed in history

or an era that was imagined overhead

and then a story (usually based on love)

can be worn with one's own body when crossing

or a soul crossing (the soul penetrates into the body of people of that era)

In the second case, many things can be changed

such as age. Gender ... can be different from the original protagonist

, but the only constant is the thought of the original protagonist

, that is, the protagonist can look at the problems of other times with the eyes and ideas of modern people

(usually the era he crossed is backward than modern times)