Trixie Tibble

 Tibble is the mother of the Tibble Twins. She has never been seen on the show, but has been mentioned multiple times throughout the series.

One of the main things revealed about Trixie is in "D.W.'s Backpack Mishap," where it is revealed that Trixie makes frequent trips to places around the world.

Arthur

 * 127a. "D.W.'s Blankie"
 * 606b. "D.W.'s Backpack Mishap"