BBB is an invincible anti-Servant robot of annihilation that we planned on putting into chapter three.
It was supposed to have been made from parts and pieces of old arenas, but ended up just being an easy to make alternate color version of BB. To differentiate BBB from BB, we made BBB the size of a Gundam. She would have probably made for a pretty disappointing boss. However BBB was supposed to be more honest about her feelings than BB, so that could’ve been pretty cute.
Passionlip said that “BBB’s not cute” and supposedly crushed her for that reason, but perhaps it’s best not to say the real reason.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA