You see, every living thing, every grain of sand on a beach, every star in the sky, is linked by an intricate web of causality. If you can comprehend that web in its entirety, down to the movements of the smallest subatomic particles, then you can predict the course of events and alter them.
~ Karen Nadler

Karen Nadler was a scout for the 2nd Massachusetts Militia Regiment. She was captured and harnessed by Skitters, However when no longer under the effects of the harness she clearly showed that she turned her back on humanity and now serves the Overlords.

After the death of her Overlord commander, Karen was appointed by her masters to take his place, overseeing all military operations in eastern North America, including destroying the humans, the rebel Skitters, and the Overlords' interplanetary enemy, the Volm.

She is shot and killed during the third season finale.