
Eva is a mess. One moment she'll be friendly and relaxed, and the next she'll be threatening to beat someone's face in with no apparent provocation. And that's just one of her issues. "Eva" is a disjointed amalgamation of eight separate aspects of her personality, ranging from happy and sweet to calm and emotionless, to violent, to shy and demure, to violently possessive; each with their own way to interact with others, and each with their own
memories. It doesn't matter if she does beat someone's face in, because only the violent Eva will remember it. The rest wouldn't know what she did wrong.
The most dominant, if it can even be called such, is the reserved and scared Eva who'll hide behind someone bigger than her, and apologize over nothing. Where other facets may not even acknowledge these other persons' existence, the "dominant" Eva will hang by their side with impunity.
Secondary, or at least the most likely to take over at random, is a much more battle-ready Eva. Aggressive and confident, and always ready to pick a fight if she feels threatened, Eva in this state is a veritable badass. Or she would be, if collateral damage wasn't a common occurrence.
Further on, others will bring Eva to treat herself like a princess among peasants, affectionately call someone her "big brother" or "big sister" despite being sixteen herself, or even lay claim to someone as her own, not to mention more.
Sixteen years ago, Eva was born the second child of a middle class family. While initially rather calm and shy in her mother's arms, months in she started being aggressive. Demanding. And not at the same time. She started developing completely different tastes for each state of mind, and it was impossible to tell when she would switch between them. Getting her properly cared for would have bankrupted her family. So, they sold Eva to the SRR when she was a year old.
Of course, the scientists didn't "properly" care for her, either. They gave her the DNA of an Eevee.
"A Pokémon can only evolve into one thing, right? Perhaps her disorder will resolve with that development."
Infallible logic.
It made it worse. Over the years, different facets of Eva's personality broke off and developed into their own just as the first three did. For each one that came up, they increased the influence of her Eevee DNA. They didn't really care about curing her at this point. They just wanted to see how far they could push her, and find out what triggered the swapping.
In the end, they'd not managed to figure out the cause, nor did they figure out how to make it any better. However, they did manage to follow eight distinct faces to how Eva displayed her emotions, organized her thoughts and responded to situations.