Newman was definitely one of the most popular recurring characters on Seinfeld, serving as Jerry's antagonist on the show.  Newman worked for the post office and often was a part of Kramer's schemes.

Newman Quotes

Is that Drake's Coffee Cake?

Newman

Newman: Kramer!
Kramer: Oh! That's Newman.
Newman: I'm on the roof!
Kramer: Well? What are you waiting for?
Kramer: You wanna shoot some pool tonight?
Newman: I can't, I'm goin' to the movies.

Seinfeld Quotes

I'm disturbed, I'm depressed, I'm inadequate. I've got it all!

George

The carpet sweeper is the biggest scam perpetrated on the American public since one-hour martinizing.

Kramer