Bart is Bert's twin brother on Sesame Street (EKA: Episode 0687), a traveling salesman by profession. Although they look identical, Bart is the complete opposite of Bert in personality — he's cheerful, overbearing, and laughs a lot, a similarity shared with Ernie.
Bert once left to visit his brother, inciting Ernie to call him during the night about frivolous details such as being thirsty. In 2003, Bart sent Bert a book for his birthday in Episode 4034, which Bert preferred to read quietly in observance of his special day. Bart made a return appearance in 2017, redesigned as a modern hipster in Episode 4733 (performed by Martin P. Robinson).
Bart was presumably named after his (and Bert's) Uncle Bart.