Now listen, I adore Nero. Absolute icon, fabulous Camp Queen!!! She's a joy in almost every form that she appears in. She's always giving pussy pussy cunt cunt meow meow purr purr diva!!! Both of her Beast forms have great designs (obviously not counting the loli form) , Last Encore was an enjoyable watch and not anywhere near as bad as it's worst critics paint it as. And the stealth Last Encore / Arcade collab event for FGO was great!
.................... That being said, if you had to pick only one character to be the lead heroine / female face of the Fate Extra series, and you aren't going with Sakura/ BB, then the only sensible choice is Tamamo no Mae.
And I have very good evidence to backup that argument!
We know that Twice is like this super narrative foil for Hakuno, Twice's original servant was Tamamo. His new servant Saver is Shakyamuni, Shakyamuni and Tamamo are both manifestations of Vairocana.
Kiara the principal antagonist of CCC besides BB, is from a cult of Esoteric Buddhists who worship Amaterasu/ Vairocana via Dakiniten who is Tamamo's higher self /older incarnation (and is also the source for Tamamo's Witchcraft Skill) Kiara is herself a very deliberate inversion of Saver and is the only other candidate we know of for certain that qualifies for that class.
Benzaiten/ Saraswati who is a component of Meltryllis and Durga who is a component of Lip are also both goddesses considered to be manifestations of / syncretized with Amaterasu by different schools of Japanese Esotericism.
Narratively and dramatically I just feel like it has the biggest impact if Tama-chan is Hakuno's Servant.
