Character Analysis
Cyrus is the Papa Bear. He's a one-legged ex-soldier who takes his brief military career very seriously. He destines his favorite son, Adam, for the army—whether Adam wants it or not—and lets his other son Charles tend to the farm or whatever. All of the nation's military higher-ups take Cyrus very seriously, and Adam is just about the only one who sees through his dear old dad for what he really is: a fraud. It eventually comes out that Cyrus probably did something shady because when he dies he has way more money than he should have.
Big Daddy
Cyrus is the God to Adam's Adam—think of him as the ultimate patriarch who begins the whole saga. As a child, Adam sees his dad as a man who has a lot of the Hebrew Bible God's qualities: he's cold, he has a lot of rules, he's kind of scary, and occasionally he is loving but only in a tough-love kind of way. But Steinbeck also plays around with his God character by making him not-so-virtuous: one of the first things we learn about Cyrus is that he contracts gonorrhea while in the army and then is so kind as to give it to his wife. When she offs herself, he wastes no time picking up the closest barely-legal girl around to take her place. In short, Cyrus is a jerk.
But Steinbeck takes the Parent-as-God idea farther. Part of growing up, he says, is:
when it first walks into [a child's] grave little head that adults do not have divine intelligence, that their judgments are not always wise, their thinking true, their sentences just […] The gods are fallen and all safety gone. (3.2.1)
The key word here is divine (and also gods); you'll notice that there is usually a lot of god-y language used in descriptions of Cyrus. See, little kids tend to see the adults around them as infallible, kind of like gods. But as they get older, they (hopefully) start to realize that these adults are people—sometimes rotten people—just like anybody else. So basically, Adam realizes that his father isn't God.
Bad Dad
Not only is Cyrus not perfect, which wouldn't be so big of a deal—no, Cyrus did something very bad. Basically, it looks like Cyrus stole a hundred thousand dollars from the G.A.R. (the Grand Army of the Republic, an organization for Union Civil War veterans). Yeah, he stole money from veterans. Not. Cool.
So now our God-figure's reputation comes crashing down, which for Adam isn't a big deal (or a surprise, really) because he knew that Cyrus was just a big talker all along. But Charles never had that realization about their father. For him, Cyrus stayed that emotionally-elusive God-like figure, which is why he takes it a lot harder. Compare their reactions to Cyrus's crime (starting with Charles):
"That's why I hate this—his whole life gone—all gone. And his grave—they might even dig him up and throw him out." His words were ragged with emotion. "Didn't you love him at all?" he cried.
"I wasn't sure until now," said Adam. "I was all mixed up with how I was supposed to feel. No. I did not love him." (7.3.161-162)
While Charles waxes melodramatic, Adam is like meh. Which brings us to our last observation about Cyrus: also like the Hebrew Bible God, Cyrus is choosy. He doesn't even attempt to hide the fact that he likes Adam better (and for really strange reasons, too):
"You're not clever. You don't know what you want. You have no proper fierceness. You let other people walk over you. Sometimes I think you're a weakling who will never amount to a dog turd. Does that answer your question? I love you better. I always have. This may be a bad thing to tell you, but it's true. I love you better. Else why would I have given myself the trouble of hurting you? Now shut your mouth and go to your supper." (3.3.31)
So what does this mean? In the Bible, God favors Abel's gift of delicious lamb gyros over Cain's gift of organic kale (okay, we might be paraphrasing here). In East of Eden, because Adam never has to fight for his father's love and because he has to bear the burden of it by being sent into the army, it's natural that resentment should come easier to him than to Charles.
Cyrus's Timeline