Popup appears to be an assortment of multiple pop-up advertisement windows. The amount of windows can vary depending on if she's speaking or not, however she has been shown to usually have 3-5. She has no arms, and a pair of small legs. Her height is able to increase or decrease depending on the amount of windows she spawns. Her face is always on a blank window, and she has no visible mouth.
Popup's speech is controlled by the advertisements she spawns, which always try to sell advertisements 