The contrast between Duke Senior and Duke Frederick in As You Like It is a striking one. They are brothers and Duke Senior is the elder apropos of his title. The younger one stole the throne and dukedom from him. Two specifics that illustrate their contrasts are following.
Duke Senior treats the offspring of his past friends with genuine love and kindness, but in contrast Duke Frederick merely tolerates the offspring of past friends as long as it serves his purpose to do so and then casts them away. In Act II, Scene 7 Duke Senior demonstrates his quality by extending the love he knew for Sir Rowland to his son, Orlando. In Act I, Scene 3, Duke Frederick demonstrates his contrasting quality by rejecting Rosalind without notice and giving her ten days to leave her realm.
The second illustratative (illustrating) instance of the contrast between them pertains to revenge. Duke Senior had ample reason to seek revenge having had his dukedom taken from him and all...revenge against his brother Duke Frederick specifically, but he didn't seek it. Instead, he set up a nice quiet existence with his loyal lieges who joined him in Arden Forest. On the other hand, Duke Frederick was quick to seek murderous revenge when Orlando won fair and square in a wrestling contest against Frederick's champion Charles. Frederick's design was to murder Orlando and he enlisted Orlando's brother Oliver in the task.