Edith is described as the middle of Gru’s three adopted daughters.
Gru’s daughters are Margo, Edith, and Agnes, in descending age order. Margo is the eldest, Edith occupies the middle position, and Agnes is the youngest. This family structure is established in the Despicable Me films and is repeated in the character descriptions for Despicable Me 2.
Edith’s personality helps distinguish her from her sisters. She is energetic, mischievous, and often eager to get involved in Gru’s schemes and adventures. Margo is generally portrayed as the more responsible older sister, while Agnes is the youngest and is especially associated with her love of unicorns.
The wording “middle one” refers to age and sibling order, not to a visual position in a particular scene. Lucy is Gru’s wife and later the girls’ adoptive mother figure, so she is not one of the three daughters. Margo and Agnes are the other two adopted girls, making Edith the only canonical middle child.