For those who were wondering about who is newcomer, third servant in F/GO and where she come from.
Actually she is not newcomer servant but completely redesigned and she come from Fate/Stay Night. Who is she?
Her name is Stray Servant, a heroine candidate for Emiya Shirou during the plotting stage but she had to be cut from the game. She was mentioned in Fate/Side Material book. According to the info, she uses shield as a weapon and she was going to be rival to FSN!Saber.
Until Nasu & Takeuchi had green lit to bring her back and officially debut in new franchise called “Fate/Grand Order”. Her real identity and servant class are unknown at this point.
Credits to JBBS Type Moon.