Should i add my oc BlackShadow into Heathens or make a completely new book?

Let me explain who she is;
Shes a demonic entity who is very skinny and lanky (like 6'5" and 98lbs) and her skin is pitch black, hence her name.

She in the CP universe basically got kicked out of the mansion because of how violent she is and Slender was sick and tired of cleaning after her and dealing with her shit so its like "bye bi tch"

So she has a strong grundge on all them, but she'll defend Toby to his grave since he reminded her of her dead little brother

I have a lot more but those are main facts but
Should i oR???