on the day of the grand reopening of the Pizzaplex, Issabelle was killed by someone, Roxy was the closest animatronic, so Issabelle gave her a personality chip with her blood on it,
over 100 years later, after Cassie's attempt to find Gregory, Issabelle had been adopted by someone who was rebuilding the Pizzaplex, Issabelle had snuck into the RUINs, and was assisting the animatronics, and building one...
but, some of William's blood ended up on her, and William's extreme emotions caused the blood to make it so that Issabelle would stop aging
which caused the springlocks to go off, Issabelle knew he would come back, so she ran away
after Elizabeth was killed by circus baby, and Michael had either run away or been arrested for killing Evan (it's unclear), William finally confronted the animatronics, but would have to get in the Springbonnie animatronic, Issabelle punched William from behind,
(hence the 2 shadow animatronics)
And she immeditly began plotting revenge, she was furious with William, she would talk with the withered animatronics, and the toys, and even Golden Freddy, (WHO IS POSSESSED BY BOTH CASSIDY AND EVAN AFTON)
Then, in 1987, Issabelle figure out that William's victims, who she had known, weren't dead, they had come back to life inside Freddy, Bonnie, Chica and Foxy, and were killing the security guards, she figured it out about a week before William did...
He kept her a secret from Michael and Elizabeth, fearing that they would take it the wrong way, Issabelle would continue to live inside Freddy's locations...
William Afton was still mourning, then he found a girl named Issabelle living in a box behind Fredbear's, and he couldn't help but want to adopt her...