Gwendolyn "Gwen" Tennyson is Ben's ten-year old cousin (fifteen in Alien Force, and sixteen in Ultimate Alien [the two share the same birthday]).

In the original series, Gwen shares a love-hate relationship with Ben, insulting him with dry wit and sarcasm most of the time, and like Ben, she was very immature, in part due to their shared birthday. She annoyed Ben a lot, prompting him to pick on her. She uses her laptop to research any strange people and/or creatures they might come across, 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 non-canon episode "Gwen 10", when Gwen gained the Omnitrix instead of Ben and proved to be far more adept and responsible in its use than he initially was.

Overall, her most notable skill is her innate, if latent, power and ability at magic. Having within her what Charmcaster calls a magical aura (which is bright blue in the original series and shades of pink, magenta, fuchsia and varying shades of purple in Ben 10: Alien Force and Ben 10: Ultimate Alien), Gwen is capable of using various magical artifacts, charms, talismans, spell books and spells. When using the mystical and magical luck-giving Charm of Bezel, she was able to activate it instinctively, whereas Hex did or could not. Later on, when she takes and uses his staff, she was able to make it fire a light yellow-colored energy blast. According to Charmcaster, only someone of Hex's skill should have that power and ability. On two occasions, Gwen has the Charms of Bezel to become a superhero by the name of Lucky Girl. At first, her power and ability drew from the luck-granting Charm of Bezel, which innately gave her complete perfect luck. While wearing the charm, any action she took would result in a 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 greatly increased and strengthened her natural abilities to superhuman levels rather than just affecting her luck. Both times, Gwen was forced to retire when the Charms of Bezel were destroyed. However, in a twenty-year alternate future, thirty-year-old Gwen goes to the mystical interdimensional realm of Bezel to receive a new set of charms. She does not have the Keystone of Bezel in her possession.

In Ben 10: Alien Force, her magical powers and abilities have greatly strengthened and increased to even higher power levels, and she is easily able to control and manipulate pure pink glowing energy from her magical aura to create energy blasts, shields, barriers, and other structures, like hammers, protective helmets, battering rams, beams to grab and hurl enemies, stepping stones to carry herself and others through the air, and safety nets; turns out that Gwen can also synchronize her martial arts styles with her mana manipulating abilities. It is also revealed that her paternal grandmother Verdona was an energy being known as an Anodite from the distant planet Anodyne, a free-spirited alien species who possess the exceedingly strong and powerful power and ability to manipulate and control a supernatural force-like substance called life energy most commonly known as mana (which can also be referred to either chakra or chi), the very source of magic itself, which explained Gwen's unusual affinity with magic. Gwen is also able to take on an Anodite energy form like her grandmother if she is under a great deal of stress. Also notable is that over the course of the later series, she has two character designs. Her blue dress was featured in the first two seasons of Ben 10: Alien Force, and then a later red-coloured redesign appearing in Ben 10: Alien Swarm when the gang goes to Missouri, later used in the third season of Ben 10: Alien Force, and recycled later on Ben 10: Ultimate Alien.

Gwen is shown to be very intelligent and knowledgeable. Before discovering her dormant innate magical and Anodite powers, she would often use her "brains" to help Ben. She is shown to be a bit of computer and tech wiz as well. She has her own laptop which she uses to research enemies she meets. She is straightforward and very brave despite the circumstances. Gwen is a very well student, especially in episode "It's Not Easy Being Gwen", where shows that Gwen takes her education very seriously. Showing to come home late at all hours of the morning and having to wake up at 5:30am in the morning, when waking up so early she does a number of things before going to school. Such as jogging, practice French, school, and afterschool karate classes. It’s also presumed that Gwen goes to private school since it shown that her usual clothing is actually a uniform at her school.

Gwen appears in all five video games based on the series, although she is only playable in "Ben 10: Protector of Earth" and "Ben 10 Alien Force: The Game". She also appears as an NPC in Cartoon Network Universe: FusionFall.