(Fanart by Nao Nuitao on facebook)

This girl has stuck around since the beginning. Helping kids like her, who didn't get the chance to live, find new life. And even after her dad tried to set her free in FNAF 6, she just said "sorry dad, I still have work to do." The lines of the puppet in UCN show Charlie is strong, at least that's how Cassidy sees her, mature, a protector, someone who won't let William do more damage.

This is my personal theory but I don't think Charlie is going to rest until Cassidy rests and William is dead, Cassidy is still so angry and bitter against William, making him suffer and live on through all of this, Charlie wants what her dad wants, for this to end, but Cassidy won't let it so Charlie has to try and steer her into letting him go. I think Old Man Consequences in UCN was Charlie speaking to Cassidy, trying to tell her to give up, let William die and let the devil punish him.