Welcome to my first look into the lore of HSR I will be doing this every once in a while, mainly because the lore of HSR is a big one with many stories of people we will never meet. The first story we're taking a look at is the story of a Diver name Frebass who wanted to dive in IX. IX an apathetic Aeon, believing existence is absolutely meaningless, so doing anything is worthless. They don't interact with other Aeons and their existence is a mystery. Just being near IX's form can make one feel numbed and dejected. IX is the Aeon of Nihility, a being so mysterious that if you approach it, you will disappear into nothingness.
But this didn't stop Frebass from wanting to dive into IX.

Frebass and a Nameless talked, while it was meant to be a moment of happiness. The Nameless asked Frebass if she is still going into IX, clearly worried for her friend since getting close to IX can be dangerous but going into it is clearly a death wish but Frebass is determined as she want to walk a path further then Akivili the Trailblazer

Frebass has now enter IX, tho thinking it was a good idea to remove the magnetic needle from the compass, the compass can only point to one direction which is down. With nothing but her ship, food, water and her compass she descended into IX ready to tackle what IX had in store for her.

During her descend things quickly turn from good to dangerous rather quickly, during her final moments she still decides to go deeper in IX knowing she will never make it out. During her descent she started to suffer from memories lost from the beginning of her descent to the point that some memories like simple thing, like forgetting her name. Tho Frebass had her diving suit which was equipped with multiple emergency support, descending into IX was still proven to be dangerous. But sadly Ferbass days were already numbered the moment she enter IX in fourteen days and nights Ferbass as sadly passed away in IX.

During her final moments before she passed away Ferbass saw a spark of crimson and with that spark she remembered the time with her Nameless friend with the long sword cooking marshmallows one last time before Ferbass left to dive into IX
Nameless: "Never thought I would meet anyone similar to me. You've walked so much further on this road than me.
Therefore, you will surely walk with me till then end, right?"
Ferbass: "Of course. Our end has already been determined..... However, just like you said -
Even though I may turn into a shallow puddle of dead water in the end, there is still a lot I can do on my journey toward that moment.
Therefore, no matter what, I have to go try it-
- Because I will walk on a road deeper and
further than Akivili's!"
"Ferbass time of death-unknown"
"Cause of death-Diving into the depth of IX"
"Ferbass a Nameless who wanted to go down a path further then Akivili's the Trailblazer a path that Akivili's would never take."
Eventually her diving suit was discovered on another planet. Tho it belong to Ferbass her diving suit was never the same as multiple crimson like flowers cover the suit while emitting a form of erosion on the diving suit.

Some believe these flowers can only be found at the bottom of IX. Bringing into question how these flowers can even survive in IX.
Sadly that is where the tale of Ferbass the Nameless ends, tho she got what she wanted, at the end of it, she sadly passed away tho what if there was more to that story?
The Nameless with the long sword, we never got her name but what if we already know her name.
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This is where I will separate the story from my theories. The Nameless with the long sword is Acheron.
Near the end of Penacony 2.0 story, we find out that Acheron is an Emanator but to what Aeon is she getting her powers from, taking a look at her path confirms she is the Emanator of Nihility. The fact that she draws power from the Aeon of Nihility say a lot of things because Nihility can cause several negative effects on a person but being the Emanator of said path could be more dangerous then beneficial.

Acheron proven to suffer from memory loss quite often, maybe too often since she tend to forget things after it already happen. This line of text is different for both genders.

In this version she reminded of a old friend who stood shoulder to shoulder, since we're a Nameless ourselves she probably thinking of Ferbass the Nameless.

In the male version she reminded of someone who she cross blades with.

More so her memories lost it a very big issues for her since she'll eventually forget her entire life.

Now if we got back to the line. "The vast void looms near the end of the vivi memories, yet she suddenly sees a dash of crimson blinking briefly into existence in the center of the pitch-black world."
A dash of crimson is very unusual especially in IX but knowing who have crimson on them is Acheron.
Acheron tears aren't normal, her tears are crimson color for unknown reasons.

Overall that my theory I could be wrong, but considering how Acheron is shaping to be someone we can't trust it would be apparent that things will go south.
Overall that my first look writing something like this, I do enjoy the lore of the game and I'll write more stuff like this. But one thing for sure this was a Doomed Yuri to begin with.
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Feb 15 2024 edit
So before I posted this Acheron model been leak but this time in a different form, from what I understand from her kit Acheron will have a second form like Jingliu, more so she have the crimson color on her especially the Flower on her neck.

People like to believe Ferbass was Acheron Kiana and Acheron was Ferbass Mei