The Kidnapper

The Kidnapper is the central figure and main antagonist in the Five Nights at Feathers franchise. He is responsible for the many disappearances of children at every known Feathers location. He is mentioned in Five Nights at Feathers and Five Nights at Feathers 2, and his backstory is more explained in Five Nights at Feathers 3. The kidnapper does not appear in Five Nights at Feathers 4, but is mentioned, and theories are still spawned about he or she.

First Murders
The Kidnappers first known location where he started to kidnap was at Cardi’s Burgers and Fries. His signature method was that he reprogrammed the Cardi animatronic to lure children. After the first disappearance, the robots began to malfunction, and the place closed down.

Becoming a employee
While this is partly speculation, after Cardi’s Burgers and Fries closed down, the kidnapper presumably became a employee for Feathers Entertainment to gain access to the Cardi animatronic and other kids. His next big strike was at the first Feathers Pizzeria, where he kidnapped 4 children (presumably killing them), and their souls were used to haunt the animatronics. His next kidnappings were at Feathers Pizzeria, the second location after the first one closed down. The three children he presumably killed went on to haunt the toy animatronics. After this the events of FNAFE2 take place.

Final Kidnappings
However, the Cardi suit was decommissioned and sent to the Feathers Entertainment Storage Facility, making the kidnapper not able to use it, making him not able to kidnap. However, the classic animatronics were retrofitted and remastered for the new Feathers Pizzeria, and their souls continue to haunt them. It is unknown what happened to the kidnapper after this, but he was arrested, as shown in a newspaper clipping article at the end of Five Nights at Feathers 3.

Trivia

 * While the kidnapper presumably has no lore relation to the upcoming fourth game, more of what he did and how he got the children’s souls in the animatronics will be revealed, as said in a post by RedMinecrafter77