Della is sister of Donald Duck, mother of Huey, Dewey, and Louie (with an unknown duck), the daughter of Hortense McDuck and Quackmore Duck. She was born around 1920.
Della was created in 1937 by Al Taliaferro and Ted Osborne in Donald Duck sunday strips in 1937, In the original strip, Della sends her children Huey, Dewey, and Louie to live with Donald Duck, in the the strips Della was called "Cousin Della", in the animated short "Donald's Nephews" (1938) she is called "Sister Dumbella", in the family tree taken from a 1950 Carl Barks sketch, she is called Thelma.
Della appears as a child in Don Rosa's The Life and Times of Scrooge McDuck and was also included in a family tree made by Rosa. In a Dutch comic book published in 2014, written by Evert Geradts and illustrated by Maximino Tortajada Aguilar, Della is shown as being a pilot, that after an accident, he cannot return to Earth.
Between 2016 and 2019, the Dutch Donald Duck's comic book published stories where she and Donald Duck are children.
Although in English her name is Della, in other countries (like Neltherlands) Dumbella is still used, she is still called Dumbella, in the episode Last Christmas! from the DuckTales 2017 series, it is said Dumbella is a nickname Donald gave her and it makes her angry.
Della made her first animated appearance (in the short, as in the newspaper strip there is only a letter written by her) in the 2017 DuckTales reboot, where she also appears as a pilot, she ends up stuck on the moon and her children are raised by Donald Duck, she returns to Earth and finds her children and Donald living with Uncle Scrooge.