Hey as the professional FNAF person could you explain remnant and agony? I'm kind of confused about what it actually does since it seems to cause all kinds of weirdness in the books.
Alright, so basically in the world of FNAF, emotions aren't just contained in the body. If you feel an emotion particularly strongly, it can linger and infuse itself into nearby objects. Agony is the most powerful lingering emotion, and typically appears as a sort of black oozing substance.
Remnant is interesting. It's described like this in Fazbear Frights:
In FNAF AR, there are two kinds of Remnant. The normal kind, and dark Remnant, which summons Shadow Bonnie. I'm pretty sure dark Remnant is tainted by evil or Agony in some way, whereas the normal kind is just the general nonphysical-energy infused kind.
Remnant can be present without a spirit fully attached. It's a "Remnant" of something intangible, like a memory, emotion, or a spirit. We can see this with novel Charlie, as she doesn't have the original Charlie's soul, but rather is the product of Henry's emotions during her construction.
Additionally, Remnant can be used for healing, power, and giving/anchoring life. It can cure illness and injuries, increase the power of an entity, and keep someone from dying. I'm 99% sure Remnant is the reason why Scraptrap has a lot more human bits than Springtrap did; William would have regrown human bits as he wandered around finding ways to get Remnant to recover.
Remnant reacts differently at different temperature, and while it was theorized by William to be neutralized with high heat, that either doesn't actually work, or the destruction of Remnant doesn't lead to a soul being William to move on. A spirit can exist without or outside of a vessel, but interfacing with living people in that case is much harder. Remnant (afaik) is the anchoring force between a spirit and their vessel.