Celia, as lovely and supportive as she is, is certainly overshadowed by her vivacious cousin, Rosalind (one of Shakespeare's most brilliant creations). Celia's own father, Duke Frederick, admits as much, saying to his daughter, "She is too subtle for thee... She robs thee of thy name, and thou wilt show more bright and seem more virtuous when she is gone" (act I, scene 3, lines 74-80). Rosalind is never "gone" in As You Like It; she clearly dominates the play. Celia is no match for her. 'Tis no slight to say so, however, since no one (including Touchstone, Jaques, Duke Frederick or Orlando) is her match.
Celia is an example of the Shakespearean "best friend" or "second-fiddle." Although she is Rosalind's cousin and a princess, the Bard relegates her to the same position as Bianca to Katherine (The Taming of the Shrew), Nerissa to Portia (The Merchant of Venice), Emilia to Desdemona (Othello), or -- more aptly -- Hero to Beatrice (Much Ado About Nothing). With Rosalind, Celia is not -- as is Hermia with Helena (A Midsummer Night's Dream), Syvia with Julia (Two Gentlemen of Verona) or Viola with Olivia (Twelfth Night) -- on equal dramatic footing.
At the end of act IV, scene 1 (lines 207-209), Rosalind -- full of love, life and adventure -- eagerly awaits her next encounter with Orlando. She rhapsodizes, "I'll go find a shadow, and sigh till he come!" Celia's response: "And I'll sleep."