A secondary character that doesn't always get his due, Jerry's Uncle Leo appeared in 15 total episodes. His name is never revealed, only referred to as simply Uncle Leo. In an episode where Leo signs for a package of Jerry's, he actually signs it "Uncle Leo". In another episode, Jerry mentions Leo to Kramer. Kramer is initially confused, but upon remembering he says, "Oh, yeah. Right. Uncle Leo. Forgot his first name." He often brags about his son Jeffrey who works in the parks department (and is never actually seen on the show). When talking to people, he has a tendency to grab the person's arm and pull them in close - one of his many uncomfortable mannerisms.
(thanks to joeshea for inspiration)