He forces kids into doing hard labor to please adults, making them work in the hot sun while he sits back and enjoys strawberry ice cream. Although it isn't certain if he is an adult himself or a child that loves to torture, signs show that he is a child.
Negative Man wants to control every aspect of peoples' lives. He even went to the point of creating a new language. Although, the language is new and therefore isn't developed enough for anyone to speak.