In "Death of a Salesman", Arthur Miller was attempting to create fully rounded characters and explore their psychological reactions to the failure of the American Dream, which Willy Loman pursued with all the unrelenting optimism of a salesperson.
Charley is the neighbor of the Loman family and in many ways represents an American ideal. He is hardworking, successful, kind, and generous, even offering Willy a job. Unlike Willy, he raises his children to study hard in school leading to success in their careers. He is the role model of the play, especially in the way that he is clear-sighted and pragmatic, rather than a dreamer like Willy.
Linda is Willy Loman's wife and she represents the virtue of loyalty. Her understanding of Willy and their situation is grounded and reasonable. In some ways she is a perfect wife, forgiving Willy his affair, encouraging him when he becomes despondent, and keeping the household afloat. In another way, we might describe her as an enabler, feeding and sustaining Willy's eventually unsustainable life due to fear of what would happen to her without Willy. From a perspective of feminist criticism, one could argue that she is very much a victim of patriarchy, as in a society in which she could have had her own career, she would not have been placed in a position of dependence on Willy.
Happy in some ways represents the worst of Willy's traits, following his path of blind optimism and being in thrall to the American Dream, but lacking Willy's moments of insight and unlikely to be capable of the sort of self-sacrifice we see in Willy at the end of the play. Happy's compulsive womanizing and self-aggrandisement, as demonstrated in the episode where he leaves Willy at the restaurant, shows Willy's failure as a parent and the hollowness of ideals focused on material success.