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 could drive away.
Billy also has a brief cameo apparance in "Net Gains".
Billy looks similar to Lily, except he has a tall yellow poofy tuft of hair. Also like Lily, he only wears a diaper.
He is based on Lily's prototype character design, where she had a poofy tuft of hair.
His Polish name is Bartuś from Bartłomiej (Bartholomew).