Andrew Mulligan is Little Bill's best friend and he loves to have fun. Andrew is a 5-year-old boy. When Andrew's family moved two doors down from Little Bill, they became fast friends. Andrew is as good at playing make-believe as Little Bill, and both boys share a love of adventure and Captain Brainstorm. Andrew has two older cousins, 11-year-old Rachel and 13-year-old Kevin, who sometimes visit and play with Little Bill, April, and Bobby. He likes Captain Brainstorm like Little Bill does
Andrew is a red haired, freckled boy who is Little Bill's best friend. Andrew and Little Bill have many things in common - they both like Captain Brainstorm, playing construction, etc. In Copy Cat Little Bill and Andrew had a quarrel and Andrew called Little Bill a copycat, but soon learns his lesson and apologizes to him. Andrew has a pet dog named Farfy. Andrew is sometimes seen with his cousins, Kevin and Rachel. He lives two houses away from Little Bill.