There's a bunch to unpack. O_O. It's gonna be lengthy.
Whether Saya is "real" person or an imaginary figure representing Aya's self-destructive impulses and choices hardly matters considering Saya barely identifies as a human, with the sole desire to be a tool for others. (I get major end-stage Metamorphosis vibes from her)
Saya is self-destructive and seems content with that fate, not wanting more than to be used and tossed away when her usefulness has been expended.
Aya hasn't resigned herself to the same fate yet.
Aya is caught between wanting better and lacking that same self-esteem she wanted Saya to have to challenge the status quo and tread a new path in life.
Aya values Saya's looks and heart. To see those wasted so frivolously challenges the value system and belief system Aya has to have to yearn for a better life. "If good and beautiful things don't end up in good places, then even if I become better than I am now there's no hope is there?"
Despite wanting Aya herself, she refrains from having s*x with her even though Aya would likely have obliged because she saw herself as dirty, unworthy and put Aya on a pedestal. Only she realized that she was using Saya the same way the others were from Saya expressing her only worth being "an eraser" (which is a tool to literally hide people's mistakes (some might suggest that erasers remove/erase mistakes, but when you write with a pencil, the force applied will always leave marks on the paper) Aya unintentionally uses Saya to distract herself from her own feelings of worthlessness). Which encourages Aya to desperately separate from Saya and desire to "become stronger".
We see Aya of the future, more accomplished and likely with more self-esteem. She denies Saya's claim of being an eraser and instead describes her as the most beautiful. This action mirrors her previous behavior of putting Saya on a pedestal but instead of this coming from a person frustrated with their worthlessness, it comes from a position of value. By finding her own worthiness, she gives value to her friend. Though she obscures her face through this assessment, so it's unclear whether she truly believes she's in a better place. As her back turned toward us seems so...lonely. Especially since she expresses her regrets.
Personal thoughts:
I'm definitely not strong enough to watch someone I consider a friend sacrifice themselves to everybody around them for benefit to themselves let alone join them in their slow descent into nothingness. I would have had to leave for my own mental health.
There was a certain stuffy hollowness resounding in my chest reading the self-destructive actions of Saya who has no qualms about her fate. It was like watching an innocent child smile as they jump into a pit of hungry wolves, get gnawed at and then comes back all bloody to hug you without a care to their wellbeing. You just wanna scream at them hoping your words get through to them "please care! please take care of yourself! Why do I care more about you than you yourself?"
Someone smarter could have written this in fewer words hehe