She is the chief interrogator and executioner for the Church Investigation Commission, who came into the human realm from Ente Isla to check on reports about Emi's alliance with Maou. Unknown to Maou who she is, she becomes his next door neighbor and eventually became close, being a good cook she would drop by in Maou's apartment to prepare meals for everyone. She knows little about the human realm especially in Tokyo that she is having difficulty going around the city and understanding people's sense of fashion.
It seems, from the start, that Maou and Lucifer knew who she was.(Source: Animefreak)