I can't think of any significantly different explanation of how the two could be not be the same.
In actual cases of Dissociative Identity Disorder (formerly Multiple Personality Disorder) it's not uncommon for distinct alters/personalities to share memories of one another or of the core/original identity (even if the core is unaware of the others), but to not be affected by them as "theirs." Similarly, DID tends to produce very different alters with wildly divergent traits, ranging from personality to even things like allergies. So, it's not entirely unheard-of for two "personalities" to be even more different than Amena and Milliene ver.2 are. (And, presumably, Amena and Milliene ver.1.)
Why doesn't she use her psi ether to fuse her mind to Milliene's?
As she says here, "I lack the ability to maintain a hold on conscious control for very long." Possibly such a thing would be too difficult or time-consuming for her at this point. Or possibly Mill1 never bothered to learn that sort of thing. Or, as in many actual cases of DID, maybe she'd rather maintain her individual self, even at the cost of less control. (I mean, would you be willing to be fused with someone you kind of look down on, and risk having their traits? Especially if, since Mill2 is the "dominant" alter, there's a possibility that Amena could be diminished or even obliterated. I know I wouldn't want to take that chance.)
She appeared after Milliene's mind was destroyed by a demon. That's not suspicious at all.
I thought the destruction was her
overloading her Psi ETHER to block Xantrich's attack.
The sound effect when she takes over sounds like a low laugh, which is an unusual choice for multiple personalities, but normal for demonic possession.
I always thought of that SFX as a breathy noise, like when you fog up glass, to accentuate the "oooooo, woogety!" goings-on of the transfer.
She claims to have Milliene-1's powers, but she's never used them, even in battle - or maybe that was Amena's subtle influence?
Or, again, she might not have been able to take over and prod buttock. It may even be particularly difficult in a battle situation, if Mill2 is trying hard to focus and stay on top of things, thus maybe having a death grip on the mental situation.
Same general thing for the next point; she might be unable to pop up and be all, "Hay guise, here's some exposition!" (Or, alternately, unwilling to do it, especially around other people.)
In her last appearance, she denied noticing anything odd about Lavian, even though she's obviously familiar with mind-affecting ETHER.
I figured that was a gag based on her eyes having the same mind-controlly thing going on- just wanting to make sure nobody else looks askance when she pops in, perhaps.
Maybe multiple personalities are just an effect of having one's mind shattered by dark ETHER.
That was what I assumed (well, replacing the Dark ETHER part with "overloading on Psi ETHER"). There's also the possibility that Mill1 was DID beforehand, and therefore Amena acted as a backup when Mill1's brain sploded.
A spear and a long dress - now that's an accident waiting to happen!)
IDK, my experience in LARPing was that polearms aren't that difficult to wield in long, flowy clothing. You just have to keep them high enough and the ends out enough not to get snagged.
