Okay... well since Phoenix is retired I can dive into a huge part of her past that I skipped over... which actually dives into multiple personality. But I won't reveal everything of course in case she needs to return to kick the ass of someone
But like in her beginning she was just a child, I won't go into her childhood except she mentioned in her interview that Ivy was the sister that was killed.
Now part of her past is that she was sent to an asylum, she was sent there cause she attempted to kill her mother but being so young she couldn't be sent to jail, but being how completely unstable she was she couldn't of been sent to a juvie or anything like that so they locked her up in solitary confinement in an asylum where they did unspeakable things to her. And she pretty much had to have a complete transformation in which she became "Ivy." Think Trish Stratus look.
Well anyways there would be times when the old would attempt to come out and battle with the new. I.e. it would be a battle between Ivy, being the sweet loving person that was kind, forgiving, and would still stand up for herself vs. Phoenix which we know how she is. Complete contrast of characters mashed into one battling each other for supremacy over owning the body that they were both in.
Multiple personalities involve more than just emotions but more like... an entire person involved. Otherwise he'll come off as something more than bipolar since he'll have more emotions but since its more purely emotion it'll be more along those lines since its extreme cases of those emotions coming out but not really an entire person inside.
But yeah multiple personality is interesting to write... but man alive it can be crazy and twisted.