We've already cleared Tamamo's route, so we're streaming New Game+ with a different Legendary Soul, an imperious swordswoman in a red dress. Who was she when she was alive? Who knows! The Fate series likes to genderbend, so that alone doesn't narrow it down.
May contain fanservice, bisexual flirting, existential horror, and brutal emotional violence. Also, we swear sometimes and cough a lot.