Billy is a minor character in The Loud House.
He first appears in "Two Boys and a Baby". Lincoln and Clyde mistakenly bring him back to Lincoln's house, believing that he's Lily. After discovering that they took the wrong baby, they manage to return him to the park, and swap Billy for Lily before his father drives away.
Billy looks like Lily, except he has a tall, yellow, poofy tuft of hair. Just like Lily, he only wears a diaper.