Adam enters in only briefly in Act I in the first scene. His presence is important, though, for two reasons: (1) he is the listener to whom Orlando describes his woes for the audience's benefit, and (2) Oliver's character is illuminated through Adam in that we see that Oliver is horrible to Adam and not just to Orlando. Adam's is an important role because it gives validity to Orlando's claims and heroic quality to Orlando's character. Adam's character qualities inform the effectiveness of his role.
Some qualities are inferred through Adam's silence, some through his comments. His role in the conversation with Orlando and his comment, "Yonder comes my master, your brother," indicate that his work is at Oliver's manor and that he does not see Orlando often at all; they are catching up, so to speak. His silence while Orlando is sharing his woes reveals that he is an intelligent, attentive listener and that he trusts himself to make accurate observations, since he is willing to observe Oliver's behavior when Orlando suggests it:
ORLANDO. Go apart, Adam, and thou shalt hear how he will shake me up.
Adam is a trusted ally and more than a servant to Orlando. In addition, Adam trusts Orlando, unquestioningly believing him and following his directions.
Adam's comments to Oliver show he has dignity and self-respect; he takes pride in being a servant and doesn't feel inadequate or subservient as a human being. This is inferred because his response when Oliver sends him away--as he first sends Orlando away--and calls him a "dog," is an ironical reprimanding retort that rails quietly but truly and firmly against the injustice and wrong-heartedness Oliver is revealing:
ADAM. Is 'old dog' my reward? Most true, I have lost my teeth in your service. God be with my old master! he would not have spoke such a word.
Exeunt ORLANDO and ADAM
His further qualities are that he has gumption; he has courage; he has moral integrity; he has a logical mind and draws sound conclusions from observations. In addition, he recognizes his worth, dignity and just deserts, and he recognizes that Oliver is not capable of an equal recognition of human worth (this poses an interesting irony because Oliver, the master, is here proven to be inferior to Adam, the servant). All these are revealed in his silence and his few lines of retort to Oliver: it takes all these qualities to stand up to a bully of that sort.