Len Lesser appeared in just 15 episodes of "Seinfeld," his idiosyncratic character was a key ingredient in the offbeat tone of the 1990s sitcom.
Here's an info about Len Lesser's role Uncle Leo from Wikipedia.
Uncle Leo is very eccentric. When coming across Jerry, he exclaims his catchphrase, "Jerry! Hello!" with his arms wide open (when Jerry has a Cape Fear-esque nightmare about him in "The Bookstore," Leo has the word "Jerry" tattooed on the fingers of his right hand and "Hello" on the fingers of his left one). He often brags about his son (Jerry's cousin Jeffrey, who never appears on the show) and his various accomplishments while working in the Parks Department. Leo is very sensitive about greetings—when Jerry once avoided saying "hello" to Leo on the street, he was offended and complained to Jerry's mother.It was also said that his last name was never revealed, but everyone knew him as Uncle Leo.
RIP Len Lesser or otherwise known as Uncle Leo