Who is Jerry Pershek (Pachyderm)?

Jerry Persheck (aka Pachyderm or The 'Derm) is an 'unseen character'. The principal characters talk about him in anecdotes but he is never seen on camera, even in a flashback. In fact, there was no actor cast in the role nor has any actor been credited with the role. As such, there is no image available to identify him.

However, through the available anecdotal evidence, he does have a colourful history with the Seinfeld clan.

From Fandom - Seinfeld

Jerry "Pachyderm" Persheck (also known as "The Derm") is referenced by all four major characters in the episode "The Stand In". One evening Pachyderm exclaimed that he was going to approach a woman; however, before approaching her he grabbed two pieces of pizza that were much too hot with his bare hands. Pachyderm, determined to approach the woman, held the two burning pieces of pizza in his hands until he resorted to juggling them which ultimately resulted in him throwing the pizza in the air. According to Elaine, the whole place "went crazy" when one piece landed on Pachyderm's face and the other landed on the woman's face. At the end of the episode George's now-to-be former girlfriend Daphne leaves him for Pachyderm who has apparently resorted to repeating the pizza event as a way to pick up women.
_{source: Jerry Persheck}

From the S05E16 episode's script:

JERRY: ..so uh she's just sitting there and a uh Pachyderm, you remember the derm. He says uh, I'm gonna go up to her. So we uh he uh picks up the two pieces of (wipes his brow) pizza and uh the uh and then they're steaming hot and they're burning his hands see so he... he's juggling em (does juggling motions) he's jugglin em, jus throwing them up in the air and just as he gets up to her down they go. (Swallows and takes a breath) Well we all just lost it. (Fulton not laughing, stone faced) It was really, really, funny.
_{source: 'The Stand-In' script}

Then later in the same episode the 'Pachyderm pizza' story receives some reinforcement from Elaine's knowledge of the events.

[Setting: Jerry's Apartment] (Jerry and Elaine)

ELAINE: You should have told that story about Pachyderm dropping the pizza.

(Buzzer)

JERRY: I told it. (Answers the buzzer) Ya?

GEORGE: (on the speaker) Ya
...
_{source: 'The Stand-In' script}

It gets rounded out to include Kramer as one who is familiar with the events.

[Setting: Jerry's Apartment](Jerry and Kramer)

JERRY: Fulton's wife told me it's all my fault. She said since my visit he's taken a turn for the worse.

KRAMER: Did you tell him the Pachyderm story?

JERRY: (yelling) Yes I told him the Pachyderm story!
...
_{source: 'The Stand-In' script}

The episode ends with the culmination of the Pachyderm pizza slice story's build-up.

[Setting: Back at Monk's] (George and Daphne at the booth nearest to the door)

DAPHNE: George, first let me just say I've never been with a guy who was so committed to commit. I mean it's so rare in men these days an, that's what makes this all the more difficult.

GEORGE: (happily) Difficult?

DAPHNE: The other day, after work, some girlfriends and I went to a bar for some drinks and there was this crazy mishap and I wound up meeting someone as a result. George...

GEORGE: (acting disappointed) Oh, please don't.

DAPHNE: Uh, I'm sorry. I'm afraid the worst of it is it's someone you know. Jerry Persheck.

GEORGE: Pachyderm?

DAPHNE: Heh, he was carrying these two pieces of pizza...
_{source: 'The Stand-In' script}

I believe the whole 'Pachyderm' side plot was designed by the writers to further demonstrate George's persecution complex and self-perceived inadequacy in the pursuit of a mate.