Ring Ring: Eiyuu Kishi to Isekai no Otome

Three years ago, a university student named Shiori took care of a knight who was strangely transported to Japan. Although he managed to return to his world, but this time, for some reason it was Shiori's turn to appear in an unfamiliar world. With no one to rely on, Shiori meant to meet with the knight, but in this world, he had become a hero... (Source: Shousetsuka ni Narou, translated)

Ring Ring: Eiyuu Kishi to Isekai no Otome began as a web novel on Shousetsuka ni Narou under the title "Ring Ring" (リングリング).