- "Though I know I should be wary, still I venture someplace scary! Ghostly hauntings I turn loose...Beetlejuice, Beetlejuice, Beetlejuice!"
- ―Lydia says her own poem in order to enter the Neitherworld
Lydia Deetz (voiced by Alyson Court) is Beetlejuice's best friend. She lives in the fictional New England town of Peaceful Pines with her lovely mother Delia and her father Charles. She owns a black cat named Percy. Lydia goes to Miss Shannon's School for Girls. Among her schoolmates, she is friends with two girls named Bertha and Prudence (their last names were never given in the series). Lydia's rival is Claire Brewster (voiced by Tara Strong), a beautiful, rich, pompous blonde who is always mean to her. Lydia is unlike other humans and, despite her beauty, she likes all things creepy and spooky. Possibly for this reason, she is the only human (at least in the show) who knows about Beetlejuice and the Neitherworld. She often goes there and back by saying his name three times, and can also call him to her world and send him back in the same way. Lydia serves as the voice of reason to Beetlejuice, who adores her and would do anything to make her happy. Although the series includes a number of small callbacks to the film, the exact backstory of how she and Beetlejuice met in this iteration is never clearly revealed. The series begins, in "Critter Sitters," as the pair are about to celebrate the one-year anniversary of their first meeting.
- Name: Lydia Deetz
- Profile: Social Outcast
- Status: Still Living
- Catchphrase(s): "Deadly-vu!"
- Identified by: gravity-defying hair; infinite patience; apparently stunted sense of smell
- Her exact age is not given, although in The Big Face Off she makes a reference to being in the seventh grade. Assuming time passes in the show's universe as it does in real life, she would therefore be in her mid-teens by the time of the series finale.
- Even though she is the co-star of the animated series, there are five episodes in which Lydia doesn't appear.
- In the episode "Vidiots," she identifies herself as "Lydia 'Joystick' Deetz." This has led to a fan theory that her middle name is Joyce, which is plausible but unconfirmed.
- In "Robbin Juice of Sherweird Forest", it's revealed that she's violently allergic to roses.
- According to Beetlejuice, she's the only person he knows that is "nice" in ways he can actually stomach. She's kind to all animals, even insects and worms; in "Out of My Mind," he observes that no one is nicer to worms (or to him) than she is. He even has a shrine to her inside his mind, indicative of just how much he reveres her.
- He also describes her, in the episode "It's a Wonderful Afterlife," as "the greatest person I know."
- In "Oh Brother," he explicitly states that he would do anything for her.
- Claire makes life miserable for Lydia on a regular basis, although the misery is certainly tempered by Beetlejuice's influence. He dislikes Claire's general attitude, but despises her for how she treats Lydia, and frequently offers to enact bizarre revenge pranks on her. Lydia is tempted, but insists on keeping her revenge artistic rather than personal; she enjoys letting Claire worry about how she might retaliate. This doesn't always keep Beetlejuice from acting on his own, however.
- Like in the live-action movie, her father calls her "Pumpkin," and as promised in the film by him, Lydia has a darkroom in the basement.
- She says the main reasons BJ is her best friend is because he makes her laugh, he knows 101 ways to eat beetles, and (in his own words) he's "the ghost with the most."
- Lydia and Beetlejuice are often "shipped" by many fans of the cartoon; according to several of the writers, this was invoked deliberately in the show. Several characters they encounter are uncertain as to the exact nature of their relationship, but it's clear to everyone who meets them that Lydia is the one individual who matters the most to Beetlejuice.
- In "It's a Wonderful Afterlife," Beetlejuice experiences an alternate reality in which he never existed. Without his influence, Lydia is a lonely, depressed, and painfully shy girl who longs for just one friend. Despite it being against the rules, Beetlejuice presents himself to this version of Lydia and manages to make her laugh, promising to show her even greater things in the future. However, he's then informed by his 'guide' that Lydia can't be part of the new un-reality he's wished for himself; realizing that he would leave her completely miserable without him, he immediately cancels the wish and returns to his regular afterlife.
- Lydia only scares Beetlejuice once, in "Scare and Scare Alike," by saying his mother called and was on her way to visit for a whole month.
- Lydia may have inspired a lot of other goth cartoon girls: Sam Manson (Danny Phantom), Gwen (Total Drama series), Triana (Venture Bros.), Carrie (The Amazing World of Gumball), Marceline (Adventure Time), etc.
- In "Bewitched, Bothered and Beetlejuiced", she and Beetlejuice masquerade as witches to save her pet cat Percy from the witches.
- For her film counterpart, see the live-action version of this character.