I like the idea of Polly having a dark spirit surrounding her for the first ark.
Ok, so like, at the beginning of the series, she's really agitated and grumpy, looks like she hasn't slept in a long time. She's let her depression get to her, and because of that, she's got a dark entity latched onto her that makes her even more agitated. She's stopped taking her medication and stuff to prevent it, cuz, what's the point?
Then Roxy comes in and they kinda fling themselves into this haphazard relationship. It takes Roxy finding out the truth for Polly to calm down and really let things sink in. She knows she's been avoiding taking care of herself, and finally has to confront the problem. So, they talk it out and get rid of the entity using swiggity after they summon him for the first time, and then they work together to work on Polly's problems.