Russell Dalrymple (Bob Balaban): “Not yet.”
Marceleno: “Little Yerry Seinfeld.”
Dang, I have three, but I’ll just give these two, since they’re always a pair: Cedric and Bob, those two guys who stole the armoire that Kramer was guarding for Elaine in the “Soup Nazi” episode, beat up Kramer in the “AIDS walk” episode, and finally appeared at the end of the “Puerto Rican Day” episode to once again menace Kramer.
“WHO?! Who does not want to wear the ribbon?!”
(everyone’s overlooking a certain woman who had a foundation named after her, a foundation whose meetings George was bound to attend…)
I was going to nominate Little Yerry Seinfeld himself. Nothing in the OP says the characters have to be human, eh?
Quote: “Bawk!”
Raquel Welch
Someone got Slippery Pete.
I submit: Prickly Pete.
(is this a good time to point out the “search this thread” button in the upper right. It shouldn’t bug me that people do flat out repeats, but whattyagonnado?)
It wasn’t the urinal…which is what made it far, far worse.
-Joe
Jerry’s girlfriend Sophie, who had some mysterious “tractor incident”.
George suggested that she had her thumbs cut off in a tractor accident, so the doctors replaced them with her big toes!
I thought of another one!
The Pig-Man that Kramer helped to escape from the hospital!
I’m a bit amazed no one has gotten this one yet so here goes:
Susan Biddle Ross: “If you don’t want to marry me, George, just say so.”
Marla the Virgin from the episode “The Contest”.
This far and nobody has mentioned the one recurring character that appears or is mentioned in every episode:
Superman
Mine is the unnamed owner of Maid to Order (it’s a pun).
Mandlebaum - Three generation’s worth!
“All aboard the pain train!”
Kramer’s mom. Sheri North (RIP)
Mr Hayman, George and Jerry’s high school gym teacher:
“Can’t Stand Ya!”
And of course Rabbi Krischma:
“Elaine, often times in life there are problems, and just as often there are solutions.”
Noel, George’s girlfriend…
Noel: I AM breaking up with YOU.
George: You can’t break up with me I’ve got Hand!
Noel: You’re gonna need it.
Rachel, Jerry’s girlfriend in the infamous “shrinkage” episode (and a couple others).
Not a quote by her, of course, but. . .
George: Do women know about shrinkage?
Elaine: What do you mean, like laundry?
George: No. Like when a man goes swimming. . . Afterwards.
Elaine: It shrinks?
Jerry: Like a frightened turtle!
Elaine: Why does it shrink?
George: It just does.
Elaine: I don’t know how you guys walk around with those things.
"Ven I was a little girl in Polland I had a Pony!"
I want to say the character’s name is Manya (?). A great aunt or something of Jerry’s, who dies after Jerry says he hates ponies and everyone that has them.
This is one of my favorite and most often used quotes, because I drive a Mustang and always refer to as “my Pony” and always with a Polish accent 
Bettte Midler