You were a simple adult who knew too much about Afton's plans, so he killed you and stuffed you in a suit making you the most powerful animatronic that lived

Long story short you and Afton along with Henry made the pizzeria together, only to find out that after Afton's youngest child's dead he killed 5 kids and maybe more. You know this because he told you about it, leading you to almost tell Henry and the police but before you could, you were stabbed in the back and head by Williams Afton. Only to be stuffed in a suit and hidden away only for performance emergencies only, you were withered with the rest of the old animatronic as you were the first to be awoken puppet