BB is an Advanced AI originating from the Advanced AI Sakura Matou in Fate/Extra CCC.

Originally an Advanced AI tasked with managing the health of masters, she went berserk due to a bug. As a cancer that invades the digital world, she seized control of the Far side of the Moon.

BB makes the Holy Grail War participants abide by rules, but she herself is a personification of rule breaking and illegal modifications.
While she could destroy the world on a whim, she is an underclassman type devil heroine who places all her effort in showing off to her senpai (master) as an energetic, precocious underclassman.

(Source: TYPE-MOON and Fate/Grand Order Wikia)

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