Slow and steady, and usually drunk and singing—that's Seth Beck with. An elderly working-class groundskeeper (a fancier term for gardener) who's been with the Mannons for 60 years, Seth's one of the few characters we see who isn't super rich or terminally insane. That's not to say he doesn't have his own problems—he drinks like a fish and he's a racist—but Seth does deserve a little attention in spite of these flaws and what is probably a ridiculously small bank account.
Seth represents part of the staple of Greek tragedy known as the chorus, a group of actors—usually with one taking the lead—who give the audience valuable info and comment on what's going down in the play. The chorus does all the heavy lifting and the grunt work setting things up for us viewers. Making Seth clearly working class was a pretty smart move on O'Neill's part when you think of the chorus's role.
Seth does everything the chorus does. If it weren't for his little off-the-record chat with Lavinia in Homecoming about Brant, Lavinia might never have guessed what was going on and we wouldn't know all about the Mannon Family's sordid past and seriously messed up present. And when he tells Lavinia she needs to get married and get herself long gone from the house that hate built, that's O'Neill's version of the chorus commenting on what's happened in the trilogy so far. Seth's always just a little too obedient. Because he's a servant, he really can't do anything to control the horrible actions of the other, more major characters, which would have made for a totally different and less bloody piece of theater.