Celia plays a very important role in As You Like It. She helps to define Rosalind, both in her strengths and her weaknesses. She serves to illustrate one of the forms of love Shakespeare is exemplifying. She provides the initial heroism that Rosalind can only adopt after she has transformed to image to that of Ganymede. Celia provides the dramatic vehicle for explaining the backstory of the play: she explains the events that occur years before the play opens. Also, her conversation serves to let the audience know Rosalind/Ganymede's thoughts of and feelings toward Orlando.
... let me stay the growth of his beard, if
thou delay me not the knowledge of his chin.
It is young Orlando, that tripped up the wrestler's heels and your heart both in an instant.
Nay, but the devil take mocking: speak, sad brow and
Alas the day! what shall I do with my doublet and hose? ... Did he ask for me? Where remains he?
How parted he with thee? and when shalt thou see him again? Answer me in one word.
One of Celia's most crucial roles is setting the example for and teaching Rosalind to be brave and courageous, which Rosalind learns to do fairly well once she is in man's clothes, a device Celia did not need in order to find her courage. Yet, there is one strong objection to saying Celia is the most important and crucial character in the play. That objection is that aside from helping Rosalind/Ganymede express herself about and set up her pretence with Orlando, Celia has relatively little to do after they reach the Forest of Arden. It seems Celia is so taken with a domestic shepherdess's and cottager's life that she has little time to spare for Rosalind, who now seems less worrisome to Celia.
In actuality, despite Celia's very significant role and influence on Rosalind, the most important character is Rosalind, regardless of her sullen, fearful faults. Rosalind represents the hero of As You Like It, Orland is not the hero, though Frederick and Oliver are certainly the villains. If you analyze the dominant role Rosalind has before she is exiled (when she is Celia's focus of attention) and analyze her behavior toward the pastoral citizens of Arden Forest, you will see that there she takes the hero's role in solving problems, ending disputes, setting wrong behavior right and bringing about a joyful resolution. For these reasons, Rosalind is the most crucial and important character although she would never have amounted to anything without Celia's love, teaching, encouragement, and good example of selfless courage.
No: let my father seek another heir.
Therefore devise with me how we may fly,
Whither to go and what to bear with us;
And do not seek to take your change upon you,
To bear your griefs yourself and leave me out;
For, by this heaven, now at our sorrows pale,
Say what thou canst, I'll go along with thee.
Why, whither shall we go?
To seek my uncle in the forest of Arden.
Alas, what danger will it be to us,