Agent Cole

Nothing else needed to be said
It's actually because of Cole that they figured out Bucks' name in the 2nd book, because they bumped into each other and immediately knew who each other were, Cole is FBI and has never really gotten along, however he ties along to the events in the 2nd and 3rd game. Unlike bucks, he shows his face... But not much is known.

Correction
Cole has a device that if he gets close to an animatronic he can plant on them. The device can only be used once and upon Cole's death, it will fry the animatronic, K.Oing both of them.