*Gwendolyn "Gwen" Tennyson, formerly known as Gwen 10, is an alternate "what if" version of Gwen Tennyson who found the Omnitrix instead of Ben.

In Gwen 10, the episode took place at the beginning of summer, much to Ben's surprise when he found out that he didn't have the Omnitrix and had somehow travelled back in time. At the camp site where Ben had originally found the Omnitrix, he was shocked to find that Gwen had reached the crash site first and that the Omnitrix had latched on to her wrist. Ben was visibly annoyed but attempted to teach Gwen. However, he slowly realized that she had greater control over the aliens that he had at first. Gwen turned into Heatblast and was able to blast her name into the sky with flames, which Ben admitted took him a month to learn. Ben later turned her into Grey Matter and messed with her.

Gwen later defeated one of Vilgax's Mechadroid as Diamondhead just as Ben had. However, she used her knowledge of refraction to fight more effectively. The next day, she brought Ben to a bowling alley and played as Four Arms, scoring four strikes in four different lanes before being captured by Vilgax. The Omnitrix was removed from her wrist and attached itself to Max.*