In the past life, she was Moroha's younger sister. They held forbidden love for each other. But in the present, Moroha doesn't remember anything of the past and Ranjou was determined to make him remember considering that they can now marry because they are not blood related.
From another past life, she was Moroha's right arm and companion. When the two first interact, she kissed Moroha on the lips and said that she mistook him for someone else. Urushibara acts like she doesn't know anything about their past life but she plans to make him remember using her own methods.