Xivu Arath was always the most attached of the siblings. Best example? When Savathûn leveraged her to strand Oryx in the Deep, Xivu Arath ended up doubling back and assisting him in getting out. Oryx was the fanatical devotee who never considered an alternate to the logic of the sword, to the extent that he fully accepted he may very well die. Savathûn was never committed in the first place. Always with a backup plan, self serving. Xivu Arath followed Oryx, doubted when he died and was radicalised in fury when Savathûn betrayed them.

I think she can be both, but we're just seeing the parts of her coming out the gate swinging on Titan and how hard Oryx's death hit her.