Long story short, on Game Theory's most recent video, he concluded that Kelsey, in the most recent Fazbear Frights book, is the Vengeful Spirit, previously known as Cassidy. The question arises about why the name is different. Here's my idea:
Cassidy IS still the vengeful spirit, but comes back, with a new name and the look of a prior victim to lure others to Freddy's and they die. Why does she want it? Well, to further her revenge. In UCN, there are quite a few animatronics that either canonically share a soul (The main four, as an example, with their versions) or have no known kid's souls (BB, Toys, Simulator animatronics, etc), so how about the souls being used for two things. A new form and a new soul to use to punish William.
What about the body? Without straight hair? Well, it's like putting anything in gears or, in this case, springlocks. Along with other interruptions would lead to the hair being messed up and no longer straight (the hair Cassidy has).
Why the new name? Well, the form is male. My idea is that Kelsey is the name for when she's using a male form while Cassidy is for a female form.
Additionally, about the hair, it could easily be another Golden Freddy Suit and a separate victim. I mean Freddy's/Fredbear's is a franchise with multiple stores. So I think each building, during the Fredbear era, had a pair of Springlock suits

Comments (3)
Well thought out.
Your theory is plausible
Reply to: Muffin_Cat
Thanks