Acheron is an alternate version of Raiden Mei that was unable to save Kiana, and was forced to kill Kevin alone. In her Cleave across transient world trailer we see her kevin's version of PROJECT STIGMA
They hid a little Easter Egg at 0:57. While it has Galaxy Ranger written under Acheron on the left side, on the right side barely visible is "Self-Annihilator" hidden in the smoke. With everything we've learned about her backstory and IX, I admit I'm slightly anxious now as to what the hell is going to happen in the future around Stelle/Caelus. With the Path changing mechanic of the Trailblazer, it's only a matter of time before we get a Nihility Trailblazer. That means at some point Stelle/Caelus is going to hit a bottom so hopelessly deep that they not only draw the gaze of IX, but they accept and embrace it.
