Character Analysis
Samuel is the patriarch of the Hamilton lot, but he is so much more than that too. An Irish-American immigrant, an intellectual, an inventor, a terrible businessman, and the owner of the worst piece of land in the Salinas valley, Samuel's only ambition in life is to be nice to everyone, which he does very well. He becomes good friends with Adam and especially with Lee.
The Anti-Trasks
The Hamilton plotline in East of Eden seems to run parallel to the Trask one, and occasionally they intersect. But why have them around at all? They don't exactly fit into the whole Adam-Eve, Cain-Abel allegory.
Instead, the dual-plotline thing invites us to think of the Hamiltons as contrasts to the Trasks: they have a huge, well-established family, they are merry and social, and they aren't haunted by any biblical curses. At the basis of this contrast are Adam and Samuel. First, the basics: Adam is rich, Samuel is poor; Adam has the best piece of land, Samuel has the worst; Samuel's wife is devout as devout can be, Adam's wife is a whore. You see what we mean?
In fact, the way the novel introduces Adam sets him up in contrast to the Trasks. This occurs when the narrator is talking about how people in Salinas acquired their land. First, he talks about people like the Hamiltons:
When people first came to the West, particularly from the owned and fought-over farmlets of Europe, and saw so much land to be had for the signing of a paper and the building of a foundation, an itching land-greed seemed to come over them […] The early settlers took up land they didn't need and couldn't use; they took up worthless land just to own it. (2.2.1)
And then we get a description of the second class of people:
While many people came to the Salinas Valley penniless, there were others who, having sold out somewhere else, arrived with money to start a new life. These usually bought land, but good land […]
Such a man was Adam Trask. (2.2.3-4)
But there is another thing underlying this whole parallel-families setup: while the Trasks are allegories for biblical characters, the Hamiltons are based on actual people. Yup, that's right: the Hamiltons were the very real family of John Steinbeck, though it's hard to say where facts end and fiction begins. Steinbeck did have a grandfather named Samuel Hamilton, who patented a lot of things with no success, but the events depicted in the story are clearly fictional. So even if Steinbeck just wanted to include a testament to his own beloved family, Samuel and Co. have plenty of good reasons to be in this book in their own right.
Dearly Beloved Plot Device
Samuel plays a particularly important role in the novel. One of his main functions in the story is to give advice… and provide commentary… and be a mentor. It's a lot of work. Think of it this way: when something isn't quite right at the Trask ranch, Samuel is the one who lets us know.
He's the one who figures out that Lee isn't the person he seems to be at first (could you imagine Adam being the one to find that out? It just wouldn't happen), he's the one who witnesses the odd birth of the twins (and tells us that something is seriously off), and he's the one who reminds Adam that he has to be a character in the story and not just a zombie. In short, we need Samuel around.
You could think of Samuel as playing the role of the audience in this story: he witnesses all of the Trask madness and provides the appropriate reactions and comments. Look at what he says to Lee after the birth of the twins:
"This birth was too quick, too easy—like a cat having kittens. And I fear for these kittens. I have dreadful thoughts gnawing to get into my brain." (17.3.88)
A passage like this sends us, the readers, a very simple message: trouble is coming. See how that works? We as readers need to be told things somehow.
It makes a lot of sense that Samuel would be the one to tell us something like this, too, because he is named after the biblical prophet Samuel (who has an entire book of the Bible named after him, which is pretty neat). Just like the biblical Samuel had God-sent visions of the future, so too does Samuel Hamilton. Like this one:
"There's a black violence on this valley. I don't know—I don't know. It's as though some old ghost haunted it out of the dead ocean below and troubled the air with unhappiness. It's as secret as hidden sorrow. I don't know what it is, but I see it and feel it in the people here." (13.3.106)
Yikes—sounds pretty ominously prophetic to us. Thanks for the warning, Samuel.
Samuel's Timeline