Della has rarely appeared in any stories, so information about her is largely unknown. According to the Eiser Award-winning comic book series The Life and Times of Scrooge McDuck, Della and Donald are twins. 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.||”|
— Disney Comics Mailing List, 1994, Don Rosa
The relationship with her sons and Donald, and why she leaves them with her brother, is debatable. 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 children with her brother just before she left for a space trip.
Other times implicated that Della was around to take care of the boys and did miss them when they were with Donald; 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. This is also carried into the 2017 version of DuckTales, where it is shown that Donald raised the boys on his own, with Della being mysteriously absent.
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.
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 was given to her by Barks. However, in Don Rosa's Duck Family Tree, first published in 1993, she was given the first name Della again. Della later appeared 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, 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 throughout 2017 in the "Donald's eerste" (Donald's first) line of comics. Showing Donald's first experience with computers, traveling and many other things.
Della makes her animated debut in the series. 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's in their adventures. Unlike most portrayals of the character, her hair color is now white as opposed to blonde.
Based on flashbacks in the show's tie-in comic, this incarnation was brave, ingenious, and eager during adventures, although her impetuous side led her, Donald and Scrooge 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.
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 with the only entry being 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 Clan McDuck crashes on Ithaquack, Dewey and Webby investigate the Temple of Heroes for clues as 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.
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.
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.
- "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 of Gladstone, she can be seen 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 comics occupation.
- Della Duck at INDUCKS
- Storyboard sketches and background information of the Donald Duck comic covering Della's fate at the blog of Disney comics creator Evert Geradts