Is he just like Chica from FNAF1, where he can disable your door jammer? Except, he disables your flashlight? Or can he actually kill you? He got in the security room and he just stood there with the sign and the balloons, but he didnt kill me.
According to the five nights at freddies wiki: Balloon Boy, abbreviated to BB on the Custom Night screen, is one of the new minor antagonists in Five Nights at Freddy's 2.
BB is a small, round, human-like animatronic with peach-colored skin, as well as large blue eyes, an orange triangular nose, red-brown hair, and a toothy grin. He wears a red and blue vertically-striped shirt, with two buttons down the middle. The propeller beanie he wears on his head bears the same pattern. He also wears blue pants and brown shoes. His cheeks, like the "Toy" animatronics, are rosy as well.
In his right hand, he holds a red and yellow striped balloon, and in his left, he holds a sign that reads "Balloons!".
BB starts in the Game Area and will eventually come to The Office via the Left Air Vent. With each of his movements, the player can hear him either saying "Hello", "Hi", or laughing. BB will not show up on any cameras except the Game Area and the Left Air Vent, making his location hidden.
He will not enter The Office if the player puts the Freddy Mask on while he is in the Left Air Vent's blind spot. If the player pulls up the monitor while BB is in the Left Air Vent's blind spot, he enters The Office, laughing constantly. Once inside, he disables the Flashlight and the Left and Right Air Vents' blind spot lights. This leaves the player susceptible to being attacked by Foxy, as well as being unable to check each vent's blind spot for other animatronics.
So, he is only there to make foxy come after you, and he does not kill you.