Police say they initially interviewed the boy as a witness to one of the fires and when his statements didn't match with what they knew investigators grew suspicious.
The boy's foster father, who requested his identity to be hidden, spoke outside his home in the same neighborhood where the fires were set to vehicles and houses.
"I would have never saw this coming," he said. "I'm sorry for their loss. I'm just as shocked as they are. I can understand their frustration."
The string of fires and car break-ins began Christmas Day when Lisa Morris' family's SUV was set ablaze in the garage, with the flames spreading to the house.
"The question is always why," said Morris. "Why? Why would someone do this? Why would a child do this?"
No one was hurt in the fires, which terrorized neighbors in Plainfield's Arbor Creek subdivision for weeks.
Police called the incidents crimes of opportunity. All of the fires were started in unlocked vehicles using lighters and other materials found inside.
"His motivation was basically to see how big the fires could get," said Plainfield Police Chief John Konopek. "He actually did show remorse. He absolutely wanted no one to get hurt.
"Again, you have to remember, we're talking about a 12-year-old juvenile, and that entire thought process may not be going through his head at the time."
Police say they arrested the child after investigators, who had been watching him for days, spotted him breaking into a car.
"We just asked him is there anything else really you need to tell us, and at that point he just spontaneously provided the information on all the fires that occurred," Konopek said.
Police say the boy's family has been uncooperative, a claim denied by the foster father, who says he wasn't asked to come to the station while the boy was being questioned.
He also claims the vehicle the boy was spotted breaking into was planted by detectives.
"You don't set a trap for a kid," he said. "If you notice a kid doing something wrong, you should notify their parents."
In addition to five counts of arson, the juvenile is charged with vehicle burglary and criminal damage to property. He's currently in custody and a March 27th trial date in juvenile court has been set.
