Gwendolyn "Gwen" Tennyson (voiced by Meagan Smith) is Ben's ten year-old cousin (the two share the same birthday). Gwen is the voice of reason that tries (usually unsuccessfully) to keep Ben from getting himself into trouble; Ben tends to ignore her and thinks that she has no idea when to stop nagging him. Gwen is always trying to plan ahead and calculate the consequences of their (mostly Ben's) actions, because she is convinced that Ben never thinks before he acts.
She uses her laptop to research any strange people and/or creatures they might come across. Although Gwen shares a love-hate relationship with her cousin, she sometimes insults Ben with dry wit and sarcasm most of the time, she loves him as her cousin and shows him respect from time to time.
Gwen is a capable gymnast and has had some training in martial arts. She has also shown an almost innate skill in using various complicated devices or objects, such as in the hiatus episode "Gwen 10", when Gwen gained the Omnitrix instead of Ben and proved to be far more adept in its use than he initially was.
Overall, her most notable skill is her innate, if latent, natural ability at magic. Having within her what Charmcaster calls a "magical aura", Gwen is capable of using various magical artifacts and spells. When using the magical, luck-giving Charm of Bezel, she was able to activate it instinctively, whereas Hex did not. Later on, when she gets a hold of his staff, she was able to make it fire an energy blast â€” according to Hex, only someone of his skill should have that ability. Gwen has only recently started to actually tap into her magical powers to help Ben using a book of spells she took from Charmcaster in the episode "A Change of Face". In the possible future of "Ben 10,000", future Gwen (voiced by Tara Strong) combination of physical and magical training leads her to become both a black belt and a master magician using the recreated Charms of Bezel, which makes her a hero in her own right. In "Ken 10", Ben's son, Ken, considers her as an aunt and she gives him a rock creature as a pet similar to the ones Charmcaster uses.
On two occasions, Gwen has used magical items known as the Charms of Bezel to become a superheroine by the name of Lucky Girl (bearing resemblance to the Marvel Comics character, Hellcat). At first, her ability drew from the "luck" Charm of Bezel, which gave Gwen control over the laws of probability, similar to the Marvel comic character Scarlet Witch. While wearing the charm, any action Gwen took would result in an implausible chain reaction benefiting her original intent but usually injuring Ben in the process. She later brought back her superhero identity after obtaining the Keystone of Bezel, which increased her natural abilities to superhuman levels rather than affecting her luck. Both times, Gwen was forced to retire when the Charms of Bezel were destroyed. Gwen's future self apparently learns to recreate the charms somehow, as they are an integral part of her costume.