Lucy

Lucy is Arthur, D.W., Kate, and Cora's aunt who was married in "D.W. Thinks Big." She is the sister of Jane, Jessica, and Fred and the daughter of Grandpa Dave, and also the youngest in her family. She is also the sister-in-law of David, Loretta, and Richard. Although not appearing again, she is mentioned several times, such as in "Night Fright" and "Ants in Arthur's Pants." A picture from her wedding appeared in "Grandpa Dave's Memory Album."

Past Life
Lucy grew up near Unidentified Town on Grandpa Dave's Farm (inferred as her dad Grandpa Dave is said to have lived there for a long time).

Trivia

 * In every later episode mention, she is referred to as a cousin, instead of their aunt.