According to Muppet Wiki, "Biff is a construction worker who speaks with a gruff New York accent.
His partner is Sully. Biff first appeared in Season 5 (1973-1974) and made periodic appearances through Season 30 (1998-1999).
In the song "Five Feet High and Rising", he mentions that he's married to a woman named Ethyl, but in Episode 3217, his wife's
name is Celeste. Biff often asks for Sully's opinion, but interrupts him before Sully gets a chance to even make a sound.
He also tends to ask him questions and then eggs Sully on by repeating, "Huh? Huh? Huh?", a reference to a routine by comedy
duo Burns and Schreiber. He often has trouble figuring out how to perform various tasks, and by the time he's figured
it out, Sully has already finished it. He also has a niece named Roxie Marie and four kids, one
of whom is the infant Mary-Biff.
Sully is a construction worker, and a friend and colleague to Biff, who first appeared in Season 5 (1973-1974).
Although Sully never speaks on Sesame Street, it is suggested that he is indeed capable of speech. In the song "Think of Your
Fingers," Biff recalls Sully giving him advice on how to remember the number 5. Also, in Big Bird Gets Lost, while drilling
loudly, Biff tells Big Bird what Sully says. Most notably, though, there was one time when Biff actually listened to what
Sully had to say. This was during the introduction to "This Song is For the Birds". The only thing was, Sully whispered in
Biff's ear, so we couldn't hear him. In addition to working in construction, Sully is good at playing the piano. However,
Biff is sometimes oblivious to this."