Brad is the son of Bert's brother Bart who appeared on Sesame Street beginning in its ninth season. Although Brad's limited speech and mannerisms suggest that he is between one and two years old, he is about half the size of Bert, making him appear a bit older.
His most prominent appearance was in an oft-repeated sketch in which Bert's attempts to give him a bath become hindered by Ernie's bath toy obsession (First: Episode 1134). He also appeared in Episode 1068 where Bert tries to teach him his first word (which turns out to be "Bob"), and in Episode 1198 where he grows his first baby teeth.
One of several types of love that Bert describes in a Sesame Street Pageant includes, "When I hold my little nephew, and I bounce him on my knee, I'm smiling down at him, and he's smiling up at me. That's love."