Juliette became a Hexenbiest, and was later shot by Trouble.
She then is turned into the super-soldier Eve, working for the organization Hadrian's Wall. Eve claims to have all of Juliette's memories, but a complete disconnect from her personality.
But how did Juliette became Eve? There's a brief reference in a conversation between Eve and Meisner to "doing to him [Nick] what you did to me" as an alternative to recruiting him, suggesting some sort of mind control or brainwashing, but it's not really brought up again. The few flashbacks we see seem to suggest at least some component of brainwashing by physical violence, but (in real life, at least) that usually isn't sufficient to totally alter someone's personality.
On the other hand, later statements make it sound like Eve has made a conscious decision to stop being "Juliette." A conversation at the end of Season 6 makes it sound more like "this person is dead," a la Darth Vader:
EVE: A lot of bad things happened to me when all of this started. I didn't understand it. I was scared and angry, and I did a lot of terrible things. Things I can never forgive Juliette for. But I'm not Juliette anymore, Nick. She's gone.
There may be a part of me that is like her, but it is not who I am now and it's not who I'm gonna be again.
Grimm, "Where the Wild Things Were" (Season 6 Episode 11)
So what is the relationship between Juliette and Eve? Is Eve just a Juliette who has made a clean break from her past actions and self, using a new name to reinforce that? Is she some kind of mundane or magical alternate personality created by brainwashing by Hadrian's Wall? Or some combination of the two?