As a Fusion, she must take the form of her host, Gwen Tennyson. The only differences are her Fusion Matter skin, dark green hair, red eyes, dark blue-green shirt and lack of a ponytail.
Fusion Gwen was created by Fuse to assist Fusion Ben in the space port, though she also has another fusion location. Given her assignment, Fusion Gwen locates Ben Tennyson and studies the Omnitrix/Ultimatrix Progress Rate and at the right time, she steals an alien DNA sample when Ben transforms into a transformation. However, on her way to deliver the samples, Gwen drops them and the Hero's mission is to receive and return the samples before Fusion Gwen returns to retrieve them. She then blocks the entrance to Fusion Ben's Lab to convince the Hero that she had the last sample, the Swampfire Root. After the Hero defeats her, they take the sample as it was formed into a Swampfire Nano.