How did Syndrome know about Jack-Jack?
All he knew about the family was Bob, Helen and the 2 kids. He was even surprised that they had 2 kids, so how did he find out about the 3rd one?
shareI think he probably just found their house somehow, looking for them, then quickly discovered that they had a third kid. Then that became his new agenda for revenge.
There’s even a deleted scene on the DVD which pretty much depicts this. He shows up at the Parr residence and meets that girl who is watching Jack Jack. She asks him what the “S” stands for and he says it stands for “sitter”.