I'm fairly sure a fusion of some sort has happened.
I (Phil) feel different, and I'm beginning to remember things from highschool. Specifically, Karkat's time helping us through it. I remember sophomore year, which is well known throughout the system as one of the worst years of our life, as well as when we figured out we were a system at all.
I'm not sad, as I barely knew her, and since her memories and mannerisms are still here through me, it's barely like she's gone at all.
I've had a feeling that some sort of fusion was gonna happen soon, as I had started heavily thinking about it for no reason out of the blue. If anything, I was expecting me and Pastra to fuse, but it has its own subsystem now, so that's unlikely to happen any time soon.
Our outerworld partner is taking this surprisingly well, too. I was expecting him to be upset because Karkat was one of his partners, but really now I'm like, girlfriend^2, because I remember Karkat's time with him, too.
I can see why it happened, as from what I've heard about her, we're fairly similar. We both are very girly in our own right and have similar favorite foods and even mannerisms.
I'm also not upset because this is a sign of healing. We've been on vacation, which has really done wonders for our mental health (albeit coming with the added side effect of having weird system shit going on).