Pint-Sized Slasher

The Pint-Sized Slasher was a Pre-War 21st century serial killer who wore a creepy clown mask and used a large chef knife as the signature killing tool.

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

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 mask.