Matilda McDuck is an anthropomorphic female duck.
Matilda McDuck is Scrooge McDuck's sister and the daughter of Fergus McDuck and Downy O'Drake. Always morally upstanding, Matilda was very much the consience of Scrooge, and also helped contain the most destructive trantrums of her sister Hortense.
Matilda left Scotland for America in 1902, following her brother Scrooge to Duckburg, and subsequently travelled around the world with him for several years, before leaving a morally deteroriating Scrooge and going back to Duckburg. When Scrooge finally joined her there, he behaved absolutely horrendeously towards her and the rest of their family, prompting both Hortense and Matilda to swear never to speak to Scrooge again.
Matilda then left Duckburg and went back to Scotland, where she was eventually hired by Donald as a caretaker for Castle McDuck; this is also where Matilda and Scrooge were eventually reunited in 1955. Matilda forgives Scrooge when she understands that Scrooge has regretted his behavior toward them and has changed. Scrooge explains that the reason why he didn't make amends with her and Hortense was not out of greed, but out of shame for his behavior. At an unknown point, she married the professor Ludwig von Drake.