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 to the park, and swap Billy for Lily, before his father drives away.
Appearance
Billy looks like Lily, except that he has a tall, yellow, and poofy tuft of hair. Just like Lily, he only wears a diaper.
Trivia
He is based on Lily's prototype character design, where she had a poofy tuft of hair.
