Pint-Sized Slasher

The Pint-Sized Slasher was a pre-War 21st century child serial killer who wore a sinister clown mask, and a black and white striped shirt, with jeans, and used a large chef knife as his signature killing tool.

You are able to pose as him in the quest Tranquility Lane, as you have an option to kill all residents while posing as the killer child clown himself.

Background
A myth used to scare children who misbehave, the Pint-Sized Slasher is said to appear from a dog kennel at dusk to chase the children around with a Toy Knife. The Pint-Sized Slasher has been the second-best-selling Halloween costume, topped only by Captain Cosmos.

Article
One of the newspapers shown on loading screens mentions the Pint-Sized Slasher. The full article can be accessed via the Capitol Post newspaper office in L'Enfant Plaza:

Behind the scenes
The Pint-Sized Slasher seems to be a reference to the horror movie Halloween, a film about an unstoppable masked killer who committed his first murder as a child when he stabbed his sister to death while donning a clown costume, complete with a mask.