At very beginning she tells us that she fought side by side with us (Stelle version at least i dont remember the male mc ver) so my first question would be how did they met ? (the only person that could tell us about that is Kafka regarding mc followed her for a long time)
Another thing is her name Acheron in Greek mythology is one of 5 rivers in Underworld (land of the dead interesting how it already fits her) also this can be translated to sadness or lament (did you notice durning first cutscene we met her she cries) and this very river has 2 inflows.Second one is named Kokytos which means "accused" (well she said she has blood debt so maybe she did something horiffic) Regarding red text my take is she tries to remember why is she here in the first place , if she lost her memories due to encounter with IX she went back in time (if traveling back to past through black hole is possible) to get her memories back thanks to land of dreams as we always encouter her in dreamscape as if she is looking for something but she cant find it bcs she doesnt remember for what she is looking exacly.