Who is Charles Trask from East of Eden and what is their importance?

Charles Trask is the son of Cyrus Trask, and Adam's half-brother. He is more aggressive than Adam. As a boy he is jealous of the fact that their father seems to love Adam but not Charles. Charles beats Adam severely because of this jealousy. When Adam joins the army, Charles remains on the ranch but he misses his brother. When Adam returns, there is always tension between the two men, and they quarrel. Charles sleeps with Cathy without Adam's knowledge.