The disturbed rebel with nobody to turn to
Although he may have been friendly, Plato was very introverted and shy. It's pretty obvious he lacked the experience to talk to people well. He didn't have many friends, before Jim came along. Even at the police station, he has a hard time talking to anybody, even his housekeeper/caretaker.
The most apparent trait of Plato was that he was damaged. Neglected by his mother and father, he searches for something to fill that void. He may have also been looking for attention when killed those puppies. Near the end of the film, Plato yells at Jim saying "You're not father!". Clearly the boy was disturbed in some fashion.