User blog comment:Hitsuji Mamoru/Servant Creation Room 2 : Guess of the Who/@comment-31726296-20170716021510/@comment-4802891-20170716152827

"Someone else's Buster attack"? Ah. So it's a character from the Nasuverse somehow linked to an existing character in FGO, and she is from something that wasn't made into an anime yet.

She might be from "Mahoutsukai no Yoru" or another TM stuff I'm not familiar with.