Celia is in some ways the mirror that Shakespeare holds up to the audience to show the depths of Rosalind's passions. For that reason, the fact that Celia in many ways resembles Rosalind is not surprising. The two girls have almost identical backgrounds. They are princesses, cousins, and inseparable companions, brought up together from their earliest childhoods. Like Rosalind, Celia is physically attractive, intelligent, and witty; also, like Rosalind, she has a bright sense of humor. Both girls embody the essences of the ideal heroine. Celia also shares with Rosalind a reflective turn of mind, which is seen in their discussion of Fortune and Nature (I.ii.34-59).
Celia is not, however, a carbon copy of Rosalind. Rather, she serves as a foil, a mirror, a young woman who brings out, by contrast, the distinctive qualities of the play's heroine. That she shares the same virtues with Rosalind raises her attractiveness, of course, in the mind of the audience.
Although Celia is quite able to hold her own in witty conversations with Rosalind and Touchstone, she is usually reserved in public situations; in the important scenes in which both girls are present, the scenes are dominated by Rosalind. In Act III, Scene 2, for example, Celia says nothing for almost two hundred lines, which is to be explained, in part, by the fact that Rosalind is Shakespeare's principal creation, and by the fact that throughout most of the play, Celia is not in love. In terms of stage decorum, it is necessary that Celia, or someone else, be on stage during the courtship scenes to lend a certain respectability and to keep the scenes from degenerating into burlesque. Thus, Celia acts more or less as a "chaperone" in the play. When at last she finally falls in love herself, Celia is won over immediately by Oliver; she never takes part, however, in her courtship as does Rosalind in her own spirited, frustrated, and protracted courtship. Humorously, Orlando is incredulous at Celia's capitulation to his brother's avowals of love. "Is't possible," he asks Oliver, "that on so little acquaintance you should like her . . . and, wooing, she should grant?" (V.ii.1-5).
Celia provides yet another function that is often overlooked by many modern-day audiences. She serves to remind the audience that Rosalind is an actor — that is, she is a boy who is playing the role of a girl who, in disguise, is playing the role of a young man. There is much humor in Rosalind's masquerade as "Ganymede." The epilogue, in particular, which is part of the burlesque of the play, loses much of its humor unless the audience remembers that the actor playing Rosalind was a boy in the Elizabethan productions.
Celia's role, then, is ultimately subordinate to that of her friend, Rosalind; she has the dramatically somewhat thankless part of serving as a companion rather than as emerging as a strong personality in her own right. Yet without Celia's acting as a kind of mirror to Rosalind, Rosalind's character would lose a great deal of its brilliance. Celia's friendship for Rosalind is perhaps the most striking feature of her personality. We first see her comforting Rosalind (I.ii.1-32), and later, when the tyrannical Duke Frederick vilifies Rosalind, Celia springs to her cousin's defense, absolutely unaffected by her father's unjust remarks, which are calculated to arouse her envy and resentment (I.iii.68-88). It is Celia who proposes that the two young women flee the palace and run off together. Importantly, Celia does not once hesitate to leave the comforts of the court in order to face the dangers of exile in order to be with her friend. Denied great romantic scenes in the play, Celia nevertheless shines passionately as the devoted friend of Rosalind, loyal, precise, and ever practical.