"Umm..There is one anmatronic in the backroom..*cough* cough* I am not sure if he umm..-is being used anymore,the employess call him "The Incinerator". Umm they say that he likes to destroy the other animatronics just for the fun of it and the reason for his behavior or existence is unknown" Quote from Phone Guy (FNAF1) (Night 1)
The Incinerator is kills and destroys other animatronics for this is unknown reasons.
The first night, The Incinerator is inactive and isn't destroying anything but it should also be considered that The Incinerator can also destroy the doors or lights and cameras so that the player can't use them.
On the second night, The Incinerator is active but not as much as the other ones but he is still active and roaming the pizzeria.
On the third night, The Incinerator is very active and can mistake the player for an animatronic and kill him or her.
On the fourth night, The Incinerator is very active moving very quickly and can even burn out windows in the office with his flamethrower.
On the fifth night, The Incinerator is inactive,and is also inactive for the sixth night.
Seventh night is same as the others can be set on 1-20.
"Not all animatronics are evil and can get in the office. There's one in the Parts/Service room which is pretty much broken down, which is very nice to you and destroys the other animatronics if needed" Quote from FNAF2 (Night 1)
He is as active in this game as the others