Springtrap

Thirty years after his death, Afton's flayed corpse still wears the withering Spring Bonnie suit, unable to take it off. Now known by the name Springtrap, the suit has decayed to a sickly olive color and the many holes and tears expose parts of the rusted metal endoskeleton. His mouth has rotted into a permanent wide grin, the teeth being chipped and yellowing. His human-like eyes are a pale gray and he's missing the top half of his right ear, as well as the casing on his feet.

Rare boot-up screens for the game show glimpses of Afton's almost mummified skull inside of Springtrap's mask. His mouth has been permanently speared open by metal rods and his eyes are organic, the spring-locks stabbed into his eyelids and holding them open in a gruesome fashion. What appears to be human innards can also be seen wrapped around the endoskeleton through large hole in the torso.

Prior to its withering, the Spring Bonnie suit was an optimized version of the Bonnie the Bunny animatronic, only with golden yellow fabric fur and a spring-lock endoskeleton. Judging by posters in Security Breach, the animatronic also wore a purple bow tie and had violet eyes.

Despite his sadistic and dangerous tendencies, it is implied that he didn't want his daughter Elizabeth to die, as he warned her not to go near Baby, and he did send Michael to Circus Baby's Entertainment & Rental to free her. Elizabeth also implies that William made Baby in her honor, making his daughter the only person that he may have cared about. However, Freddy Fazbear's Pizzeria Simulator implies that he has a dysfunctional relationship with his daughter, and that he is responsible for influencing Elizabeth's blood lust and sinister behavior (although that might also be because of Baby's programming corrupting Elizabeth). He also may have even wanted her to continue his murderous legacy alongside him. Besides, he knew that Circus Baby was possessed by Elizabeth, yet still allowed his employees to shock her. There have even been some theories that Afton wanted Elizabeth to be killed by Baby and used reverse psychology to do so, claiming he created the robot for his daughter but not allowing her to play with it while making her watch as she interacts with the other children.

Adding to that, in the The Fourth Closet, although a separate timeline from the games, shows that William was an abusive father to Elizabeth, going so far as hitting her and pushing her to the ground when she simply disrupted his work. Taking in the fact of how both versions of William act almost completely identical to each other, it's possible that he didn't have an emotional connection to Elizabeth in the mainline story either, rendering him to be truly heartless.

William's motives for his actions, as well as his backstory, remain unknown to this day, with many believing he became a killer because he was driven by some unknown tragedy (most often interpreted as the loss of his family), a selfish goal for immortality, or he was just a twisted and sadistic monster who wanted to see people suffer, while some believe his motivation might be a combination of the above. However, there is still no official confirmation as to why Afton became the monster he is today.