Amari was born as a result of her mother "sleeping up the social ladder". Growing up, she was constantly ridiculed for both her origin and the fact that her given name translates to "leftovers". Eventually she decided to enter the Moon Cell with the intention of gaining fame and prestige. She planned to use weaker people as pawns to strengthen herself, but she was immediately disheartened upon realizing she was one of the weakest participants there. At some point she started dating Shinji Matou.
In an earlier Holy Grail War, Amari apparently challenged Rin to a battle while on the third sanctum. However, Rin rejected the challenge, saying that Amari would lose her next battle either way and she had no reason to fight someone who would never make it to the sixth sanctum.
Amari is shown to have long pink hair with straight bangs and pink eyes and wears a school uniform with long white stockings with grey shoes.
Growing up without any skills of her own, Amari holds a strong dislike for talented people. She is shown to have a hatred for her mother due to her mother allowing herself to be used by people with more social standing. This led to her belief that it is normal for talented people to use people less talented than themselves for their own ends. She hates her given name.
During her conversation with Rin, she is shown be somewhat haughty and overconfident. She is also prideful as she was furious when Rin rejected her challenge. this is later revealed to be a facade, as she actually possesses a somewhat low self-esteem, hating herself for not being talented and falling into depression and drinking after losing her Servant
Fate/EXTRA Last Encore
Amari is a student of the Tsukumihara Academy and is in the class of the male Hakuno Kishinami and Shinji Matou. She is Shinji's girlfriend and a friend of Hakuno, sticking around with the two of them. Later on, after the last remaining Masters were starting to be chosen and the other students being slaughtered, Hakuno asks Shinji where she is, to which he responds that she is in a safe place.
She is granted the position of a Master, making it at least to the third floor. Her ultimate fate, however, is unclear. She was present on the third floor around the time Alice still retained human form, which was long before Hakuno made it to the third floor himself. Amari, or Caster in Amari's form, was also responsible for convincing Alice's "Onii-chan" to depart from the third floor; again, long before Hakuno got there.
Both Hakuno and Rin experience visions of Amari during their time on the third floor. Hakuno discovers through his visions about Amari's background, which she explains in a somewhat drunken state. Rin also has a vision of Amari beckoning Rin to fight her so Amari can advance from the third floor, though Rin coolly rejects her.
The Drama CD reveals that she was contracted to a Rider-class Servant and had made it to the third floor. They were driven away by the monstrous Alice, however, so Amari decided to assist Shinji with his city. She and her Servant are killed by Rin Tohsaka just before the city is flooded.
Nothing is shown of Amari's capacity as a magi or a Master. However, given Rin's words to her and her own acknowledgement of being untalented, its likely that she was not very skilled.