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

Susan: (pounding on Kramer's door) Kramer! Kramer!
Jerry: What's that? What's going on?
Susan: Kramer, open up, I know you're in there!
Jerry: Susan.
Susan: Kramer!
Jerry: What is going on?
Susan: You know what's going on? First, he vomits on me! Then, he burns down my father's cabin! And now, he's taken Mona away from me!

Dolores: Oh, Jerry.
Jerry: Oh...you.