William David Afton is the overarching antagonist of the Five Nights at Freddy's franchise and is heavily implied to be the protagonist of the spin-off Ultimate Custom Night.
He was a man who dressed up as one of the mascots at Freddy Fazbear's Pizza (Spring Bonnie) and used his kid-friendly disguise to lure five children into the Safe Room, where there were no security cameras. It was confirmed that William was a former security guard of Freddy Fazbear's Pizza.
After an accident where he set off the spring-locks in the Spring Bonnie costume, he becomes The Springtrap and is the main antagonist of Five night's at Freddy's 3 and secondary antagonist of Freddy's Fazbear's Pizzeria Simulator. Afton is also the primary antagonist in the FNAF novel series.