>>537953817
BB's existence is really strange even by FGO standards, because from her own canon she is a super AI living in a super advanced simulated reality, in which she is basically a god. That system is called the Moon Cell, because it's literally on the moon. In the moon, more accurately. I won't explain further. But basically her class, Moon Cancer, was invented just for her, specifically, which is why the class card for Moon Cancer is a picture of her. Other class cards are more generic.

Eventually they did add other Moon Cancers but it's one of the rarest classes and they're usually very special characters.