Purple Guy (or The Killer.) was originally the main villain in the series. Purple Guy was Katie Sandow's enemy until Vicky take over. His goal is to kill everyone and take over the series and be the main character (but The Killer is not a official character.) However, he turned out to be Katie Sandow's Fake Father, it revealed when Emily tells the history of God child's in 2005 to 2015, Katie was curious that why Purple Guy was in it but Wallace almost revealed that Purple Guy turned evil when they first became there god parent until Emily zips his mouth.
The killer was a man who dressed up as one of the mascots at Freddy Fazbear's Pizza and used his kid-friendly disguise to lure five children into the back of the pizzeria. He then murdered them and stuffed their remains into the mascots' costumes. He was arrested the following day and charged, but his current status and influence on the game's events is unknown (it is also speculated that he wasn't really arrested and the one who was caught was simply a scapegoat). The prequel states that the killer was a purple man who possible could somehow associated with the Puppet (if the Puppet had anything to do with the murders) or is even possibly one of the security guards.
Aftr that, he actually "Adopted" Katie Sandow to kill her later but luckily she escaped and he reveals he will be back.
FNAF'S 2 Song "It's Been So Long" is the song from The Living Tombstone, when Purple Guy has no god friends, he turned evil and has an idea about killing innocent children, he dress up as Freddy Fazbear suit to trick the kids and kills them with the dagger inside the backstage. Katie Sandow did not appeared in the music but she did "Singed" it. The Blue top kid was in the restaurant with his friends until he got killed from Purple Man along with them and turns into the Animatronics.