Stellula Eques ~Shokuzai no Himekishi~

One day protagonist sees a beautiful girl being attacked by monsters. When he saves her, he realizes that she is an old childhood friend. She tells him that she is actually a magical knight fighting to save the earth. They can't help but fall in love with each other. Now she must keep her sense of duty and not lose herself in his love giving up the fight. Is it still possible to save the earth ? [Translated from Getchu]