
Celia is Rosalind's constant companion and the play's dry counterpoint to romantic excess - her loyalty drives the flight to Arden, her disguise as Aliena shares Rosalind's exile, and her sudden, matching love for Oliver completes the play's pattern of paired unions.
Celia is Duke Frederick's daughter and Rosalind's cousin, and the play establishes in its first scenes that her loyalty to Rosalind outweighs any loyalty to her own father - she offers, unprompted, to give Rosalind back everything Frederick took from Duke Senior the moment she herself inherits it. When Frederick banishes Rosalind on a sudden, unreasoned suspicion, Celia's response is immediate and absolute: she will not stay at a court that has exiled the person she loves best, and proposes to run away with her before Rosalind has finished protesting.
In the forest, Celia takes the disguise of Aliena, a poor shepherdess, and largely recedes into a supporting role to Rosalind's Ganymede - a shift the play treats with some irony, since Celia was the more socially secure of the two at court. She is a sharp, dry commentator on Rosalind's lovesickness and Orlando's courtship game, unimpressed by romantic performance even as she is quietly falling into her own love story.
That story arrives all at once: Oliver, sent to the forest by Frederick to find Orlando, is saved from a lioness by his own brother, and falls in love with Aliena within the same scene he repents his cruelty. Celia's acceptance of him is reported rather than dramatized at length, but it completes the play's symmetry - cousin for cousin, brother for brother - and by Act V she is as ready for marriage as Rosalind, her earlier scepticism about love apparently no defence against it.
Celia is motivated above all by her devotion to Rosalind, a loyalty that outweighs her position at her father's court entirely, and secondarily by a dry scepticism about love that her own swift falling for Oliver rather charmingly undercuts.
Celia's wit is quieter and drier than Rosalind's, deployed mostly at Rosalind's expense as commentary on her cousin's lovesickness, but her loyalty is the most unwavering feeling in the play - she gives up her position at court without a second thought rather than lose Rosalind. Her own capitulation to love, sudden and total, is played partly for comedy precisely because she has spent the play needling Rosalind for the same thing.