ef - a fairy tale of the two.エフ ア フェアリー テイル オブ ザ トゥー

Yuuko Amamiya, a mysterious girl dressed like a nun, and Yu Himura, a mysterious gentleman who is somehow attached to the church where Yuuko first appears, are having a reunion in a church during Christmas time. Despite her attire, Yuuko is not affiliated with the church. She always appears generally out of no where, and disappears just as quickly in various places throughout the story to talk with Hiro or other characters and give them advice. Yuuko and Yu reminisce about the past and remember events of the previous year around the same time at the beginning of the first chapter of the story. Yuuko hints of events that are revealed throughout Ef: A Fairy Tale of the Two..

Source: Wikipedia