BB (ビィビィ), known as Mysterious Black Magus (黒衣の少女) in the original promotional materials, is a character resembling Sakura Matou in Fate/Extra CCC. It is unknown how she came to exist, and she is said to have created the Sakura Labyrinth (サクラ迷宮). She resides in Tsukimihara Academy’s infirmary, and she is shown introducing herself to Sakura, mentioning that she doesn't mind being called Bebe, Baby, or anything she likes. She claims to be an unusual and dangerous AI. Both Meltlillith and Passion Lip resemble her.
Source: Type-Moon wiki
She is introduced as a limited servant in "Fate/EXTRA CCC Collaboration Event" event in April-May 2017. Her class is "Moon Cancer", a new servant class introduced in the same event based on her role in Fate/Extra CCC.