Her Royal Duchess Diamond Persnickety, the First, Last and Only or simply Duchess (referred to by her full name in the pilot movie) is the overarching antagonist of Foster's Home for Imaginary Friends. She speaks with a German accent and is voiced by Grey DeLisle.
This imaginary friend is a scheming, snobby, pompous, stuck-up, vain, conceited, negative, extremely rude, short-tempered, arrogant, selfish, self-centered, and mean Picasso-like pain in the neck who is actually two-dimensional when she turns her head or body (however she is not two-dimensional in the pilot movie, House of Bloo's.) She's considered a "high-maintenance" friend, not wanting to do anything for herself, and is so lazy that she needs somebody to open her eyelids when she wakes up. The residents seem to follow her commands because she is both able and willing to scream incredibly loudly for an indefinite period of time without rest unless she gets what she wants, as was seen when she lived next door to Mac and was able to yell non stop from him going to bed to his waking up, revealing a rather spoiled and annoying side to her personality. In a few episodes, she has helped someone, but only so that she could help herself, thus for selfish reasons. For example in "Affair Weather Friends", she told Mac about Bloo hanging out with "Barry Bling" (aka Berry in Disguise) behind his back. Duchess is one of the worst imaginary friends due to her ugliness and being evil.
In the pilot, she and Terrence conspire to get rid of Bloo after the millionaire's spoiled daughter nicknamed Sweetums wants to adopt him (and, in the process, rename him "Tiffany") instead of Duchess, who was chosen by her mother and father. As punishment for trying to get rid of Bloo, she was not kicked out of the house (or sent to prison at that matter), but rather was forced to stay as she desires to leave.
Despite her desire to leave, Duchess insults almost everyone who tries to adopt her except for the rich family in the pilot showing that while she badly wants to leave she doesn't want to live in any less pampered lifestyle she would be forced to give up if she went home with a normal family (in other words, she wants to be adopted by a rich family). Everyone else desires her to leave very badly however, so much so that Mr. Herriman spends the entire budget for advertising adoption on her, even saying that he will pay a family to take her.
Duchess has yellow skin, a long neck, a beauty mark, a trunk-like nose, a mouth that is turned sideways and one of her eyes appears to be upside down.
She wears a white turban with a ruby in it, light blue and purple eye shadow, pink blush, red lipstick, green and black striped earrings, a blue and red pearl necklace and bracelet, a white dress with black triangles on it, two black bracelets, red nail polish, black and green striped leggings, and black high heels. Her appearance is based off of Picasso's good work.
- Duchess only appears in nineteen episodes, almost half of which are non-speaking cameos.
- There is a running gag where she believes she is the most beautiful in the world, only for everyone else to consider her extremely ugly.
- Her skin appears to be a more pale shade of yellow as seen in Affair Weather Friends.
- She is next to Coco and Eduardo in the picture.
- She can control Extremosaurs as seen in House of Bloo's.
- A few episodes where Duchess actually helped someone include House of Bloo's, The Big Picture, &Affair Weather Friends. But it was ultimately for selfish reasons. To be fair Bloo was no better in the latter.
- She appears at least once in every season.
- Duchess is able to smell money with her nose.
- Among the main characters, Duchess is one of the characters that never was arrested and jailed during the series. The others are Frankie Foster, Goo, Cheese and Terrence.
- Duchess's golden trading card statistics in the Foster's Home for Imaginary Friends Leapster game are:
- Despite what her golden trading card says, Duchess surprisingly bears no animosity towards Uncle Pockets, considering him "tolerable" though she doesn't necessarily like him either.
- It is possible that Duchess has a very good sense of smell.
- Although Duchess doesn't appear in the series finale Goodbye to Bloo (unlike all the other recurring characters or the main characters on the show), Her final appearance was in The Bloo Superdude and the Great Creator of Everything's Awesome Ceremony of Fun That He's Not Invited To.
- Partying is Such Sweet Soiree
- Where There's a Wilt, There's a Way (non-speaking cameo)
- Bloo's Brothers (non-speaking cameo)
- Beat With a Schtick (non-speaking cameo)
- Bloo Done It (cameo)
- My So Called Wife (cameo)
- The Bloo Superdude and the Magic Potato of Power! (non-speaking cameo)
- The Bride to Beat
- Affair Weather Friends
- Better Off Ed (non-speaking cameo)
- Destination Imagination (non-speaking cameo)
- The Bloo Superdude and the Great Creator of Everything's Awesome Ceremony of Fun That He's Not Invited To (Final Appearance)