One of the main Servants who forms a contract with the Main Character.
A young swordswoman clad in a vibrant red dress, which she claims is a man’s outfit. She is often called Red Saber to differentiate her from other Sabers.
She frequently refers to herself as the Emperor, a role that she cannot easily cast aside, and finds difficulty in speaking plainly.
She usually refers to the Main Character as “player” (sousha; instrumentalist), but uses “you” (sonata) when engaging someone as a woman (romantically).
Note that during the Holy Grail Wars the Main Character is the only one she uses “you” (sonata) for.
Nero is a self-indulgent ruler who forcefully makes her desires a reality, which she chalks up to the many talents that compose her genius. People mistakenly assess her as a tyrant because of her arrogant attitude and overly confident manner of speaking, but she recognizes that everyone has their own way of life, and won’t try to deny someone of their own path.
She uses a person’s beauty to determine whether they are good or evil.
She’s well-versed in theater, song, and various other arts, such as painting and sculpture. She uses these talents to capture the attention of the Main Character, whom she looks to with tears in her eyes and asks, “How is this? Is it not fantastic? Although it might be a bit of an acquired taste. Well? It’s nice isn’t it!?”
Her stats would not normally qualify her as a Saber-class Servant, but with her Imperial Privilege, she becomes one anyway. In truth, she is a Commander-type Servant.
Being responsible for the Regalia, the Main Character is also responsible for foreign diplomacy and fostering good relationships with other Servants, a fact that bothers the Emperor as it takes attention away from her.
“You need to spend more time with me! I’m the one bound to you by contract, so I should come first!”

At the outset of EXTELLA, under the mistaken assumption that the Moon Cell had gifted only her with the Regalia, and in line with her duty as Emperor of her territories, she attempts to quell the rising chaos in SE.RA.PH.
She is ecstatic at the prospect of fighting together with the Main Character when they first awaken, but shifts her focus to protecting him or her when she finds out the Main Character in danger of disappearing.
Her main priority becomes defeating Tamamo no Mae in order to merge the pieces of the Main Character and the Regalia back together, returning them to their complete state. Although she acts like a tyrant, internally she is trying to piece together the “why” and “how” of the situation she finds herself in, as well as the identity of whoever is behind it all. She realizes quickly that the one chosen by the Regalia was not her, but the Main Character, and comes to have doubts about Archimedes, who did not point this out to them.
The ultimate goal of Nero during this period is to mend the Main Character’s fragmented consciousness.
She will do what the Main Character thinks is right, and accomplish what she thinks is beautiful.
She will destroy the Umbral Star. She will defeat Altera as her rival. She will punish Archimedes. She will protect the Main Character and that which the Main Character wishes to protect.
And in the end, no matter what happens, she is totally convinced that her biggest competition (in securing the Main Character’s heart) is Tamamo no Mae.

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA