Eh, sheet posted. It might be a little wordy though so I may get back to it eventually.
The actual abilities of the symbiont creature are up to you to decide, if you even allow it that is.
How are the Slate Meritocratic? Also will she be able to Shapeshifter some, since she is a hybrid? Or are Slate stuck in their shifted form? In general I like it, and it makes for some interesting plot ideas, like combatting personalities and slow dehumanization. They sound like the Thing, one of my favorite movies.
Shes human, and lives in a human society, so I thought the human part would count for meritocracy. She just doesn't tell most people about her phisiology.
The slate part is a separate entity that feeds off her own nutrition (think of Parasyte: The Maxim). As for its ability to shapeshift, I mean, it depends on what kind of balance you're going for.
Basically, she has some level of control over its nervous system given that she can still walk. Since it's large and well-developed, it can shapeshift fast enough to assume vital life functions before she died, so it's a fast shapeshift although not necessarily instant.
Yes, they can shift again after they assume forms since the slate used to be a soldier form before it took on a semi-human form.
It only occupies the lower half of her body, so the rest of her can't shapeshift (for now).
I'm hoping for some plot development though so I can take the character further.