Della Duck

Della Duck, also known as Dumbella Duck and Thelma Duck, is Donald Duck's sister and the daughter of Hortense McDuck and Quackmore Duck. She is also the mother of Huey, Dewey, and Louie, Donald's triplet nephews.

Background
Della has rarely appeared in any stories, so information about her is largely unknown. According to chapter 11 of the Eiser Award-winning comic book series The Life and Times of Scrooge McDuck, Della and Donald are twins. However, when the story's author, Don Rosa, was later told that Donald and Della being twins (instead of Della being older than Donald) would raise timeline issues, Rosa said that Donald and Della might be twins, but they don't necessarily have to be, leaving it up to the interpretation of the reader :

Yes, I DO like the idea of Donald and Della being twins. But, as you say, if I simply OMIT that one comment, everything works out fine just as it is (...) Della and Donald still look like twins, and maybe they are... but I don't think anyone can trip me up on years if I don't nail it down needlessly. I think my Duck Family Tree will be in Walt Disney's Comics & Stories #600, but naturally birthyears won't be listed, and it's not possible to tell if siblings are twins or not on that tree.

The relationship with her sons and Donald, and why she leaves them with her brother, is inconsistent. In the original comic strips in which Huey, Dewey and Louie were introduced, Della was around to take care of the boys and did miss them when they were with Donald, and, despite always remaining an invisible character, would correspond with Donald via letter, telegram and over the phone; in fact, one instance showed that she was more than happy to have them back after Donald had grown tired of them.<  However, as of the 1942 cartoon The New Spirit, the nephews were listed as being adopted by Donald.

According to the Dutch Disney comic 80 is prachtig, Della was one of the first female pilots, and eventually became an astronaut, so she left her infant children with her brother just before she left for a space trip. Coincidentally, Donald already mentioned having numerous pilots in his family in the 1942 theatrical short Donald Gets Drafted ("I come from a family of aviators! My father, my uncle, my cousin..."). This is also carried into the 2017 version of DuckTales, where it is shown that Donald raised the boys on his own after Della was lost in outer space.

History
Donald's sister was first mentioned in the cartoon short Donald's Nephews (1938), in which she sent her sons Huey, Dewey, and Louie to visit Donald. In the letter, she was referred to as Dumbella Duck.

In the newspaper comic strip of November 21, 1937, Donald, who has grown increasingly tired of his nephews, receives a telegram from Della in which she says that she wants Huey, Dewey, and Louie to come back home, which they do, much to Donald's pleasure. However, the trio returned to appear in the comic strip not long after their departure, in a May 1938 installment of the comic strip. Here, Huey, Dewey and Louie's mother phones Donald, with only the following part of the phone conversation being given to the reader: "― and I have go out of town for a while, and I knew you'd love to keep the little dears!". Donald begrudgingly accepts, and his nephews immediately speed into his house on their tricycles.

The nephews' mother made her first visual appearance in Mark Worden's illustrated version of Carl Barks' Duck family tree, by the name Thelma Duck, which had been given to her by Barks himself. However, in Don Rosa's Duck Family Tree, first published in 1993, she was given the first name Della again (presumably to keep the alliteration), except she appeared on the tree as Donald's sister and not as his cousin. Della later made a cameo as a child alongside her brother and her parents in Rosa's The Empire-Builder from Calisota, chapter 11 of The Life and Times of Scrooge McDuck.

Della reappeared in the comic titled 80 is prachtig (written by Evert Geradts and drawn by Maximino Tortajada Aguilar), where she was given a new history and background as one of the first female pilots, who eventually became astronaut, leaving her children with her brother just before she left for a space trip, only to get lost in space after a faulty rocket launch. This comic was made in honor of Donald Duck's 80th anniversary in 2014 and was published in a special issue of the Dutch Donald Duck magazine.

A younger Della continued to appear in the Dutch Donald Duck comics (also scripted by Geradts) throughout 2017 in the "Donald's Eerste" (Donald's first) line of one-page gag comics showing young Donald's first experience with computers, traveling and a variety of other things.

DuckTales (2017)
Della makes her animated debut in the series. In this incarnation, Della was a pilot and traveling companion of Scrooge and Donald. She, like Scrooge, was brave, ingenious, and eager during adventures, although her impetuous and stubborn side led her group into trouble much of the time, much to Donald's dismay and worry. She also wasn't above making light-hearted jokes at her brother's expense. The mystery of Della's absence in the nephews' lives is a central story thread in the first season of the show, with the ending of the series premiere having Dewey discover a painting, showing that she was previously a companion of Scrooge and Donald in their adventures. Unlike most portrayals of the character, her hair color is now white as opposed to blonde.

In "The Great Dime Chase!", Dewey goes to Webby Vanderquack for information about Della, mentioning that Donald had never said more than one word about her and their only memento of her is a photo of Della pushing Donald's face into a cake. Sadly, Webby has little to no information on Della, as if she disappeared off the face of the earth. She mentions that Scrooge bought a mail company the day after they delivered a piece of junk mail turned up at Della's door. Thus, Dewey and Webby go to the archives at The Money Bin but Miss Quackfaster kept stalling them with so-called trials. Eventually, Dewey finds a book titled with his mother's name and the only entry is a reference number, which led him and Webby to a secret room, where they find a letter from Della to Scrooge, revealing she had stolen the Spear of Selene, which was mentioned in "Woo-oo!". Fearing the possibility that his mother betrayed Scrooge, Dewey tells Webby to hold off telling anyone until they know more.

In "The Spear of Selene!", when the family crashes on Ithaquack, Dewey and Webby investigate the Temple of Heroes for clues to Della's whereabouts, while debating whether she took the Spear of Selene for nefarious or altruistic purposes. Eventually, they meet the moon goddess, Selene, who tells Dewey that his mother was a true hero and a devoted mother, but has no idea what the spear is, though she insists that Della wouldn't have taken it if she didn't think it was important. To help the two on their quest, Selene provides them with the Sphere of Selene.

In "The Last Crash of the Sunchaser!", Dewey, Huey, Louie, and Webby continue searching for the truth about Della, after learning that the final piece is located on the plane. When Dewey finds the piece, the plane suddenly crashes into a steep peak, leaving the group to balance the plane equally. After Dewey successfully retrieves the piece, ignoring Scrooge's requests to follow him to safety, he demands Scrooge tells him what the Spear of Selene is. Scrooge explains that it was a rocket ship he built because Della wanted to explore outer space, but Donald didn't agree with it with her triplets about to hatch. Scrooge decided to build the rocket and took a picture of it, intending it to be a surprise for Della to celebrate her triplets' birth. Unbeknownst to Scrooge, Della had found out early and climbed into the rocket without him noticing. As soon as he realized this, Scrooge contacts Della warning her not to go further, as she was nearing a cosmic storm. After entering the storm, the ship got hit by a lightning bolt, causing the ship's transmission and engines to cut out, leaving her stranded in space. Scrooge sent other ships to find her, but it became too expensive, causing the program to be shut down. The kids, heartbroken, resent Scrooge because of this, and they, alongside Webby, Bentina Beakley and Duckworth, leave the Manor.

It is revealed at the end of "The Shadow War!" that Della had indeed survived the storm, and crash landed on the moon. Her television picks up a broadcast from Earth, depicting the Duck family victorious over Magica De Spell and she smiles at the sight of Huey, Dewey and Louie.

Classic comics
Although Della never appeared in the classic cartoons, she was mentioned on rare occasions throughout Disney's published comics.

A comic adaptation of the short Donald's Nephews was written into the Donald Duck newspaper comic strip of 1937, in which it was told that the nephews' father was in the hospital after a giant firecracker had exploded under his chair. In this comic, Della was still said to be Donald's cousin, rather than his sister.

The nephews briefly mentioned both of their parents while saying their prayers in a 1938 installment of the Donald Duck newspaper comic strip written by Bob Karp and drawn by Al Taliaferro.

DuckTales
Della appears in the comic adaptation of the 2017 series, specifically in stories that were set before the series. These stories feature the adventures she attended with Donald and Scrooge.

Trivia

 * "Della" was also the first name of Al Taliaferro's aunt.
 * In an early French translation of the comic adaptation of Donald's Nephews, Huey, Dewey, and Louie were sent to Donald by Clarabelle Cow instead of his sister.
 * Della made a very obscure cameo appearance in Don Rosa's The Sign of the Triple Distelfink. During a flashback to Donald and Gladstone's childhood, she can be seen in the background at young Gladstone's birthday party.
 * Della's introduction was the first time a Sensational Six member was explicitly said to have a sibling (Morty and Ferdie's mother was already introduced in 1932, but she was said to be Mickey's sister years after her first appearance).
 * In The Duck Who Never Was by Don Rosa, it's shown that if Donald wasn't born, Della would have left her children with her cousin Gladstone.
 * In DuckTales (2017), Della is shown wearing a pilot's outfit, perhaps as a parallel to her brother's sailor attire and/or a nod to her occupation in Donald's 80th-anniversary comic.