Deirdre Shannon lives in Nancy Drew's hometown of River Heights and has been Nancy's sworn enemy since they were in 1st-grade. She's slender with short, dark hair and pale skin. Like Nancy's father, Carson Drew, Deirdre's father works as an attorney, and the Shannons are a wealthy family like the Drews are.

Deirdre considers Nancy her rival and has a strong desire to prove that she's better sleuth than Nancy is, despite the fact that neither of them are professional detectives. Deidre's parents are often pressuring her into being more like Nancy, and Deirdre will sarcastically say, "Well, if it isn't the famous Nancy Drew!" whenever she sees Nancy, hinting at a strong jealousy towards Nancy's popularity.

She used to go by "Dee Dee" as a little kid, but hates the nickname now, and snaps at anyone who calls her that. She's considered a rich, spoiled, self-centered snob who dislikes Nancy and her two best friends, Bess Marvin and George Fayne, but is also shown to have a crush on Nancy's boyfriend, Ned Nickerson, who she often flirts with.

Deirdre's seen in the computer game, Alibi in Ashes, where she can be found at the Scoop Ice Cream Parlor. Deirdre's also said to throw great parties.