I suspect she is indeed what her name implies, the last living member of the Choir and/or at least one who is still loyal to the goals of the healing church.
I suspect she was sent by the higher ups in the choir chain of command to find and destroy Rom so they can make a go at stopping the ritual.
She failed but she’s continuing her search via the university library to find answers, seeing as Master William is unable to speak she has to figure things out on her own.
When you arrive she’s naturally hostile for to her you’re just another common vagabond looking pillage the school of its valuables and so in turn makes an effort to try and stop you at all costs.
My theory anyway, it’s possible she’s just a body guard and aid sent by the healing church to look after Master William and make he’s well looked after.