Celia is Rosalind's cousin as well as her good and true friend. She is Duke Frederick's daughter. She lives at the palace with Rosalind, who comes to live there are Celia's father ousts Rosalind's father, Duke Senior. Despite her father's actions, the two young women become very close. They love to laugh and make fun of the goings-on in the court, and enjoy bantering with Le Beau and Touchstone, particularly.
Celia is amused by Rosalind's infatuation with Orlando. She is troubled when her father banishes Orlando from the court after the wrestling match. She becomes outraged when Duke Frederick also ousts Rosalind, and, true friend that Celia is, she accompanies Rosalind to the Forest of Arden.
The girls know that they are in danger in such a place, and adopt disguises for their safety. Rosalind becomes "Gaynmede," a male persona; Celia remains a woman, but calls her self "Aliena" (a clever Shakespearean pun of the word "alien" for Celia acts as an alien, that is, an outsider, but she has also "alienated" herself from her father and his rule).
Celia is the less dynamic of the two female characters, and after her transformation, her role in the play is diminished. She is reduced to a go-between for Orlando and Rosalind. Her own relationship with Oliver is never directly played out, only observed in conversation with other characters.