Margaret is a dog, the mother of Jake, Finn, and Jermaine. Natasha Allegri has confirmed that she and her husband Joshua adopted Finn as a baby. The couple first appeared in one of Finn's flashbacks in the episode "Memories of Boom Boom Mountain." They found Finn after he had soiled himself, causing him to get stuck to a leaf, after he cried for an entire day, all alone in the forest. They were the first and only people to stop and help baby Finn, as all other passersby simply ignored him. They took him to what is presumably at their house and raised him as their own. Margaret appears to be caring and loving. She speaks in a 1940s American accent.
According to Finn in "Conquest of Cuteness," Margaret always knew "the right thing to say" whenever he or Jake were in a crisis. Strangely, a Two Headed Cutie tried to exploit their love for her during their invasion, pretending to be her ghost mocking them in an attempt to find their weaknesses.