Ichijo's Younger Sister

Japanese name:

一条〈妹〉

She shows up in the anime episode 7 looking for her older sister, following that episode she appears mysteriously throughout the anime. She relies on drawing on a sketchpad to communicate as she is too young to converse. In the anime series, aside from Ichijo and her sister, no one else is aware of the aliens keeping watch over Becky.

Wikipedia