Fredbear

Literally nobody

Except mangle who is theorized to be possessed by a dog or Susie's dog. But tbh I think cupcake is better at being the dog lmao

No one reminder that there censoring in there eye is malfunctioning remember

no one is possesing them, they have facial recognition in them that was hacked to kill anyone with a security vest thats why they kill you i explained it in greater detail in a comment in the fnaf_lore stream go check it out

I will check it out
Thank you

Oh

Evil chatgpt

More children.

Past security guards

Keeping in mind I have no clue about the OG games lore

Me

William Afton killed five children during the events of FNAF 2 and their spirits/Remnant attached to Toy Freddy, Toy Bonnie, Toy Chica, The Mangle and Balloon Boy, however their spirits were freed unknowingly at the end of the game when the Toys were taken apart.

Interestingly, the MCI kids (as you called them the "original spirits") possessed the Withered animatronics, and chose to stick around when being rebuilt into the Classics/OGs.