Seemingly a nice and innocent animatronic entertainer for children, he is actually possessed by the soul of a child named Gabriel and actively tries to kill the security guards of at least two Freddy Fazbear's Pizza locations, which he mistakes for his murderer.
In the original FNAF, setting the tone for the rest of the series, Freddy is possessed by the soul of a murdered child. As such, he and the other animatronics are seeking revenge against their killer, whom they mistake the various night guards for.
