Ilia is a former member of the White Fang.
Ilia's parents enrolled her in a prep school in Atlas and gave her a strict set of rules to hide her Faunus heritage, showing their desire for her to live a normal life free of discrimination. When they died along with several other Faunus in a mining accident, Ilia lost her composure and assaulted her former friends for laughing at her family's deaths.
In "True Colors", Blake Belladonna asks Ilia if her actions within the White Fang are what her parents would have wanted, causing Ilia to break in tears and say that she does not know what else to do. This, along with Ghira Belladonna saving her from death, causes Ilia to decide to leave the White Fang and help Blake defend Haven Academy.
Through the White Fang, Ilia is brethren with the two fox Faunus. They know her well enough to be aware that she has a history with Blake Belladonna and informed her of Blake's return. In "Welcome to Haven", according to Blake, they blamed Ilia and they were disappointed that she is working for Adam. However, they were revealed to have already been collaborating with her prior to this and after she had her Scroll taken. Later in "Necessary Sacrifice", the brothers assign Ilia to assassinate Ghira and Kali Belladonna and bring Blake alive to Adam Taurus, much to Ilia's dismay. She eventually betrays them after losing her will to fight.
Though they have not interacted onscreen, Ilia knows much about Adam's plans. She stored such information in her Scroll before Blake seizes it in "Two Steps Forward, Two Steps Back".
Later, in "Unforeseen Complications", Ilia denounces the Belladonnas for speaking out against Adam and proclaims only he can give the Faunus the future they desire.
In "Alone Together", Ilia admits she was jealous of Adam because of Blake's attraction to him. However, following Blake's departure, Ilia dismisses these feelings in order to pursue Adam's ideals. However, she later betrays the White Fang after Blake persuades her.
Ilia and Blake are both comrades from their old White Fang days and close friends. Ilia also holds romantic feelings for Blake, which she professes to Blake in "Alone Together", and is upset they were unrequited.
During the period between Blake's departure from the White Fang and Ilia's departure from the White Fang, Blake and Ilia are somewhat unwilling enemies. Blake wants to stop her from becoming like Adam while Ilia wants to further the goals of Adam's White Fang under the orders of the Albains. Ultimately, after Blake tells Ilia that her ways go against the wishes of her parents and Ghira saves her from certain death, Ilia betrays the White Fang and joins Blake.
In "Argus Limited", although she is saddened that Blake is leaving Mistral with her friends to Altas, she respects her decision. Ilia gives Blake an embraced and thanks her for everything before they go their separate ways.
The two first meet with an immediate confrontation when Sun and Blake catch Ilia spying on them. In "Two Steps Forward, Two Steps Back", she wounds and renders Sun unconscious with Lightning Lash to prevent him and Blake from pursuing her. Later, in "Unforeseen Complications", Ilia evades capture by Sun when she interrupts Ghira's press conference.
In "True Colors", Sun fights Ilia during the house-fire, and she keeps to defending herself. At the end of the attack on the Belladonnas, Ilia seeks redemption when she incapacitates Corsac, and when she and Sun finally evacuate, they have dejected looks. Ilia asks to join Blake to save Mistral, which earns Sun's forgiveness. He playfully pinches her arm to get even with her for stabbing him, which she apologizes for.
Neptune appears to have some romantic interest in Ilia, going up to and flirting with her, though he is unaware of Ilia's homosexuality. Ilia has a confused look on her face when he mentions he did not know she would be around.