Adam is best described in the beginning of the novel, when he is a young child:
Young Adam was always an obedient child. Something in him shrank from violence, from contention, from the silent shrieking tensions that can rip at a house. He contributed to the quiet he wished for by offering no violence, no contention, and to do this he had to retire into secret-ness, since there is some violence in everyone. He covered his life with a veil of vagueness, while behind his quiet eyes a full rich life went on. This did not protect him from assault but it allowed him an immunity.
Adam was glad of Charles the way a woman is glad of a fat diamond, and he depended on his brother the way that same woman depends on the diamond’s glitter and the self security tied up in its worth; but love, affection, empathy were beyond conception.
Living with a harsh father and minimal maternal comfort, Adam withdrew from the world; after his discharge from the army and during his subsequent time as a vagrant he pulls even further into himself:
And by now he was an expert tramp, using humility as a working principle. He was lean and sun-darkened, and he could withdraw his own personality until he made no stir of anger or jealousy.
When he is picked up for vagrancy, Adam is sentenced to build roads, and he becomes even more selfless:
And now he learned how men can consider other men as beasts and that the easiest way to get along with such men was to be a beast. [. . . ] He drew a curtain around himself. He removed expression from his face, light from his eyes, and silenced his speech. Later he was not so much astonished that it had happened to him but that he had been able to take it and with a minimum of pain. It is a triumph of self-control to see a man whipped until the muscles of his back show white and glistening through the cuts and to give no sign of pity or anger or interest. And Adam learned this.
Steinbeck was describing Adam during his time on a road gang; Adam inevitably does not stay nearly so colorless, but the damage has been done. Adam is passive throughout East of Eden: he refuses to build any further after Cathy leaves him, doesn’t name the twins until a year after their their births, and accepts with a minimum of fuss that he had been wrong about shipping the lettuce, a move all the more galling because the reader knows Adam was right. Adam does not fight well—that much is clear from his childhood descriptions—and it is clear that he never learns to.