Ilia is a former member of the White Fang.
Ilia was originally born and raised in the mining community of Faunus laborers likely working for the SDC in Mantle, with Ilia herself having likely spent a majority of her childhood growing up their together with her parents in the community of Miners who mined dust for the Kingdom.
Eventually after putting their resources together and their savings, her parents managed to enroll Ilia in an Atlas Prep school directly in the Kingdom's capital of Atlas.
Knowing full well the racist behavior common there, Ilia's parents gave their daughter strict rules in order to hide her Faunus heritage from her classmates to insure her success in life. At first Ilia was initially confused but eventually due to wanting to fit in, began to wholeheartedly follow them and joined her classmates in their racism in insulting the Faunus due to feeling and believing she was "one of them".
However her opinion changed for the worse with her parent's deaths in a mining accident. Upon hearing of the cruel words and snickering her classmates expressed, Ilia burst into a rage and physically assaulted them due to their cruelty and apathy to the sufferings of the Faunus. Since then her opinion of Atlas' upper class has taken a severe downturn, as seen where she held no reserve or remorse in actively assisting the White Fang in their various terrorist attacks and operations.
As her parents were one of the many miners working for the SDC, Ilia likely did not have a good opinion on the company due to their controversial labor laws and policies and upon their deaths in a mining accident, Ilia's opinion on them has seemingly worsened.
Later upon joining the White Fang, Ilia was seen assisting Sienna Khan and Adam Taurus in attacking one of their facilities in order to help Faunus rights showing that she had no issue attacking the company to assist in advancing the Faunus race. However despite being committed to their goal, Ilia is still shown to be uncomfortable with the excessive violence employed by some members of the White Fang.
Ilia's exact relationship with her parents is unknown, but before their deaths they were noted by their daughter to have loved and cared for her deeply. Hoping that Ilia's future would be far better than their own lives, they has pooled together all of their life savings in order to enroll her in a prep school in Atlas, hoping to give their daughter a better advantage to succeed life and give her all the benefits that they never had. Knowing of the strong racism and bigoted beliefs the Atlas elite possessed in regards to Faunus, they gave her a strict set of rules to follow in order to hide her Faunus heritage.
When they, along with several other Faunus died in a mining accident, the news reached Atlas and eventually their daughter and her friends, who then began snickering at the news at that a "bunch of animals" had died. This in turn caused Ilia to then physically assault them for their cruelty, breaking their teeth in anger.
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, 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.
During the events of the "Volume 6 Adam Character Short", it is shown Ilia working together with Adam and Sienna in infiltrating an SDC facility and assisting them from the sidelines in their mission, showing a degree of teamwork between the two. However it is also shown that despite working together in the same side, Ilia still harbors reservations and discomfort at the excessive violence against Humans employed by Adam during their missions.
During the events of the Volume 6 Adam Character Short, Ilia is seen working together with Sienna during their Raid on a SDC facility and is shown to have a good enough relationship and coordination with her to work together efficiently during missions. However like with Adam, Ilia has also shown to be slightly bothered by Sienna's extremist beliefs.
Later during the events of "Necessary Sacrifice", It was revealed that Ilia had been aware of Adam's plans to usurp Sienna's place as High Leader of the White Fang and seemingly agreed with Adams opinion that Sienna's belief's was holding the White Fang back. However it can be noted that Ilia did show a degree of guilt over her death but her faith in the righteousness of the White Fang's cause trumped this guilt.
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 embraces Blake 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.