Della Duck, occasionally referred to as Dumbella Duck or Thelma Duck, is Donald Duck's twin sister and the daughter of Hortense McDuck and Quackmore Duck. She is the mother of Huey, Dewey, and Louie, Donald's triplet nephews.
Donald's sister was first mentoined in the cartoon short Donald's Nephews, in which she sent her sons Huey, Dewey, and Louie to visit Donald. In the comic adaptation of the short, it was told that the nephew's 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' 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, she was given the first name Della again. Della later appeared as a kid alongside her brother and her parents in Rosa's The Empire-Builder from Calisota, chapter 11 of the Eiser Award-winning comic book series The Life and Times of Scrooge McDuck. In this story, Rosa was the first to mention that Della and Donald are twins.
Della was one of the first female pilots, and eventually became an astronaut, leaving her children with her brother just before she left for a space trip, as mentoined in the Dutch comic 80 is prachtig. This comic was made in honor of Donald Duck's 80th anniversary, and was published in a special issue.
- '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 told 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 1994 comic 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.